diff --git a/.github/workflows/rust-check.yml b/.github/workflows/rust-check.yml
index 55d56d61a..8289ee059 100644
--- a/.github/workflows/rust-check.yml
+++ b/.github/workflows/rust-check.yml
@@ -61,6 +61,16 @@ jobs:
         if: runner.os != 'Linux'
         run: cargo test --workspace
       - run: cargo clippy --workspace --all-targets -- -D warnings
+      # Step 1a Phase C Task 4: spec v19 §11 + §12.3 boundary invariants.
+      # Linux runner has the full mobile target stdlib (`rustup target add`
+      # in subsequent steps would handle ios/android cargo metadata too,
+      # but the script uses `cargo tree --target` which only needs the
+      # cfg-gate evaluation, not the actual target sysroot).
+      - name: Verify Jian boundary invariants
+        if: runner.os == 'Linux'
+        run: |
+          rustup target add aarch64-linux-android aarch64-apple-ios wasm32-unknown-unknown
+          bash tools/check-jian-boundaries.sh
 
   deny:
     name: cargo-deny (native)
diff --git a/README.md b/README.md
index 4d14e946b..95e9d650b 100644
--- a/README.md
+++ b/README.md
@@ -502,6 +502,7 @@ Contributions are welcome! See [CLAUDE.md](./CLAUDE.md) for architecture details
 - [x] Native agent runtime (`agent-native` — Zig NAPI)
 - [x] Git integration — clone, branch, push/pull, folder-mode three-way merge
 - [x] Canvas raster export (PNG / JPEG / WEBP / PDF)
+- [x] Rust shell — Step 1a (G1 shared Skia context) on `v0.8.0`: `SharedSkiaContext` + `NativeBackend` (Jian-`DrawOp`-backed) + `JianPointerMapper` (Jian `PointerEvent` → `ShellEvent`) + `basic_window` demo, with the multi-platform CI matrix green (macOS / Linux / Windows desktop, iOS / Android cargo check, wasm32). Spec `v19.3` FROZEN; `vendor/jian` pinned at `c4a794dc`.
 - [ ] Collaborative editing
 - [ ] Plugin system
 
diff --git a/crates/openpencil-shell-native/examples/basic_window.rs b/crates/openpencil-shell-native/examples/basic_window.rs
new file mode 100644
index 000000000..519647b6c
--- /dev/null
+++ b/crates/openpencil-shell-native/examples/basic_window.rs
@@ -0,0 +1,240 @@
+//! Spec v19 §1.2 acceptance #1 — basic_window demo.
+//!
+//! Phase C Task 4 deliverable: a minimal winit + `SharedSkiaContext` +
+//! `NativeBackend` + `JianPointerMapper` integration that paints chrome
+//! (rect / text / box outline) on every frame and translates pointer
+//! events through the Phase B Task 3 mapper. The macOS / Linux / Windows
+//! runtime is exercised by maintainers manually (`notes/step-1a-{macos,
+//! linux, windows}-manual-smoke.md`); CI only verifies that
+//! `cargo build --examples --workspace` compiles on every desktop OS.
+//!
+//! Run with:
+//! ```text
+//! cargo run -p openpencil-shell-native --example basic_window
+//! ```
+//!
+//! The window should display:
+//! - White background (chrome canvas clear).
+//! - Red filled rect at (50, 50) - 100x100.
+//! - "Hello 你好" black text at (50, 200).
+//! - Blue stroked rect outline at (200, 50) - 200x150.
+//!
+//! Closing the window must run `SharedSkiaContext::teardown` exactly
+//! once (the `Drop` impl is the safety net) and exit cleanly.
+
+#![cfg(any(target_os = "macos", target_os = "linux", target_os = "windows"))]
+
+use jian_core::scene::Color as JianColor;
+use jian_host_desktop::pointer::PointerTranslator;
+use openpencil_shell_core::{Color, Point2D, Rect, TextLayout};
+use openpencil_shell_native::{
+    JianPointerMapper, NativeBackend, SharedSkiaContext, SharedSkiaError,
+};
+use winit::application::ApplicationHandler;
+use winit::event::WindowEvent;
+use winit::event_loop::{ActiveEventLoop, EventLoop};
+use winit::window::{Window, WindowId};
+
+/// Per-frame chrome paint. Pulled into a free function so both the
+/// initial `Resumed` paint and `RedrawRequested` redraws share the
+/// exact same draw list (spec §5.2.1 frame-scoped backend pattern).
+fn paint_chrome(ctx: &mut SharedSkiaContext, backend: &mut NativeBackend) {
+    let rect_fill = Rect {
+        origin: Point2D::new(50.0, 50.0),
+        size: Point2D::new(100.0, 100.0),
+    };
+    let rect_outline = Rect {
+        origin: Point2D::new(200.0, 50.0),
+        size: Point2D::new(200.0, 150.0),
+    };
+    let text = TextLayout::single_run(
+        "Hello 你好",
+        "",
+        24.0,
+        JianColor::rgb(0, 0, 0),
+        Point2D::new(50.0, 200.0),
+    );
+
+    ctx.begin_frame();
+    ctx.with_frame(|canvas, _glow| {
+        // White background — clear the framebuffer through Skia.
+        canvas.clear(skia_safe::Color::WHITE);
+
+        // 1. Filled red rectangle (acceptance #1 chrome rect).
+        backend.fill_rect(canvas, rect_fill, Color::RED);
+
+        // 2. Black "Hello 你好" — exercises CJK path through jian-skia
+        //    `textlayout` ParagraphBuilder.
+        backend.draw_text(canvas, &text, Point2D::ZERO);
+
+        // 3. Blue stroked box (acceptance #1 chrome outline).
+        backend.stroke_rect(canvas, rect_outline, Color::BLUE, 2.0);
+    });
+    ctx.present();
+}
+
+struct BasicWindowApp {
+    window: Option<Window>,
+    ctx: Option<SharedSkiaContext>,
+    backend: Option<NativeBackend>,
+    /// Phase B Task 3 wiring: winit `WindowEvent` → Jian `PointerEvent`
+    /// → `JianPointerMapper` → `ShellEvent`. We only translate pointer
+    /// events here; window/resize/close events go straight to the
+    /// match arms below.
+    pointer_translator: PointerTranslator,
+    pointer_mapper: JianPointerMapper,
+    /// Fatal teardown / surface error captured for post-loop diagnosis.
+    error: Option<SharedSkiaError>,
+}
+
+impl BasicWindowApp {
+    fn new() -> Self {
+        Self {
+            window: None,
+            ctx: None,
+            backend: None,
+            pointer_translator: PointerTranslator::new(),
+            pointer_mapper: JianPointerMapper::new(),
+            error: None,
+        }
+    }
+}
+
+impl ApplicationHandler for BasicWindowApp {
+    fn resumed(&mut self, event_loop: &ActiveEventLoop) {
+        if self.window.is_some() {
+            return;
+        }
+        let attrs = Window::default_attributes()
+            .with_title("OpenPencil — basic_window (Step 1a §1.2 acceptance #1)")
+            .with_inner_size(winit::dpi::LogicalSize::new(800u32, 600u32));
+        let window = match event_loop.create_window(attrs) {
+            Ok(w) => w,
+            Err(err) => {
+                eprintln!("basic_window: create_window failed: {err}");
+                event_loop.exit();
+                return;
+            }
+        };
+
+        // Build the GL stack + Skia context bound to the new window.
+        let dpi = window.scale_factor() as f32;
+        match SharedSkiaContext::new_desktop(&window) {
+            Ok(ctx) => {
+                self.ctx = Some(ctx);
+                self.backend = Some(NativeBackend::with_dpi(dpi));
+            }
+            Err(err) => {
+                eprintln!("basic_window: SharedSkiaContext::new_desktop failed: {err}");
+                self.error = Some(err);
+                event_loop.exit();
+                return;
+            }
+        }
+        self.window = Some(window);
+
+        // First paint so the window has visible content even before
+        // the OS schedules a `RedrawRequested`. Subsequent redraws come
+        // through the event match below.
+        if let (Some(ctx), Some(backend)) = (self.ctx.as_mut(), self.backend.as_mut()) {
+            paint_chrome(ctx, backend);
+        }
+    }
+
+    fn window_event(
+        &mut self,
+        event_loop: &ActiveEventLoop,
+        _window_id: WindowId,
+        event: WindowEvent,
+    ) {
+        // Phase B Task 3 wiring: route pointer-flavoured winit events
+        // through Jian's PointerTranslator + our JianPointerMapper.
+        // Non-pointer events (Resized / RedrawRequested / CloseRequested)
+        // bypass the Jian path per spec §5.1.1.
+        match &event {
+            WindowEvent::ModifiersChanged(m) => {
+                self.pointer_translator.update_modifiers(m.state());
+            }
+            WindowEvent::CursorMoved { .. }
+            | WindowEvent::CursorLeft { .. }
+            | WindowEvent::MouseInput { .. }
+            | WindowEvent::Touch(_) => {
+                if let Some(jian_event) = self.pointer_translator.translate(&event) {
+                    let shell_events = self.pointer_mapper.from_jian_pointer(&jian_event);
+                    // The demo doesn't act on pointer events — it just
+                    // proves the pipeline compiles + runs without
+                    // panicking. Real widget dispatch lands in Step 1c+.
+                    let _ = shell_events;
+                }
+            }
+            _ => {}
+        }
+
+        match event {
+            WindowEvent::CloseRequested => {
+                event_loop.exit();
+            }
+            WindowEvent::Resized(size) => {
+                if let Some(ctx) = self.ctx.as_mut() {
+                    if let Err(err) = ctx.resize(size.width, size.height) {
+                        eprintln!("basic_window: resize failed: {err}");
+                        self.error = Some(err);
+                        event_loop.exit();
+                    }
+                }
+                if let Some(window) = self.window.as_ref() {
+                    window.request_redraw();
+                }
+            }
+            WindowEvent::ScaleFactorChanged { scale_factor, .. } => {
+                if let Some(backend) = self.backend.as_mut() {
+                    backend.set_dpi(scale_factor as f32);
+                }
+            }
+            WindowEvent::RedrawRequested => {
+                if let (Some(ctx), Some(backend)) = (self.ctx.as_mut(), self.backend.as_mut()) {
+                    paint_chrome(ctx, backend);
+                }
+            }
+            _ => {}
+        }
+    }
+
+    fn exiting(&mut self, _event_loop: &ActiveEventLoop) {
+        // Idempotent teardown — the `Drop` impl on `SharedSkiaContext`
+        // is the safety net, but explicit teardown gives the demo a
+        // visible "no leaks" signal at exit (acceptance #6).
+        if let Some(mut ctx) = self.ctx.take() {
+            if let Err(err) = ctx.teardown() {
+                eprintln!("basic_window: teardown failed: {err}");
+            }
+        }
+        self.backend.take();
+        self.window.take();
+    }
+}
+
+fn main() {
+    // Per-frame `#[instrument]` spans are emitted but no subscriber is
+    // initialised by default — the demo deliberately stays free of
+    // dev-only crates. Add `tracing_subscriber::fmt::try_init()` here
+    // (and drop it as a dev-dep) if you want to inspect spans.
+
+    let event_loop = match EventLoop::new() {
+        Ok(el) => el,
+        Err(err) => {
+            eprintln!("basic_window: EventLoop::new failed: {err}");
+            std::process::exit(1);
+        }
+    };
+    event_loop.set_control_flow(winit::event_loop::ControlFlow::Wait);
+    let mut app = BasicWindowApp::new();
+    if let Err(err) = event_loop.run_app(&mut app) {
+        eprintln!("basic_window: run_app exited with error: {err}");
+        std::process::exit(1);
+    }
+    if let Some(err) = app.error {
+        eprintln!("basic_window: fatal error during run: {err}");
+        std::process::exit(1);
+    }
+}
diff --git a/crates/openpencil-shell-native/notes/step-1a-linux-manual-smoke.md b/crates/openpencil-shell-native/notes/step-1a-linux-manual-smoke.md
new file mode 100644
index 000000000..39d85b363
--- /dev/null
+++ b/crates/openpencil-shell-native/notes/step-1a-linux-manual-smoke.md
@@ -0,0 +1,48 @@
+# Step 1a Linux manual GPU smoke (per spec v19 §1.2 acceptance #1)
+
+**Status**: PENDING — to be filled by a maintainer running on a Linux
+desktop with a real or Mesa software-rendered GL driver.
+
+The CI Linux runner ships Mesa llvmpipe via Xvfb but `gpu_smoke` /
+`gpu_chrome_stub_composition` are currently `#[ignore]`'d under
+`LINUX_GPU_SKIA_LOADER_TBD` (spec §3.1 mini-patch — `skia-safe`'s
+`Interface::new_native` cannot resolve GL syms from EGL pbuffer +
+llvmpipe; the proper fix is `new_load_with(eglGetProcAddress)` and is
+deferred to Step 1f). The spec acceptance #1 on Linux therefore needs
+a separate human runtime check.
+
+## Prerequisites
+
+- Ubuntu 22.04+ / Fedora 40+ / Arch with `mesa` / `libglvnd` /
+  `xkbcommon` / `wayland-client` / `freetype` / `fontconfig` installed
+  (the same set the CI `Install Linux GL prereqs` step provisions).
+- X11 or Wayland session.
+- Rust toolchain 1.85.
+- Submodules at `vendor/jian@c4a794dc`.
+
+## Required commands
+
+```bash
+cargo run -p openpencil-shell-native --example basic_window
+# After Step 1f spec §3.1 mini-patch lands, also:
+# cargo test -p openpencil-shell-native --test gpu_smoke -- --include-ignored gpu_smoke
+# cargo test -p openpencil-shell-native --test gpu_chrome_stub_composition -- --include-ignored gpu_chrome_stub_composition
+```
+
+## Expected outcomes
+
+- `basic_window` opens an 800x600 window showing:
+  - White background.
+  - Red filled rect at `(50, 50) — 100x100`.
+  - Black `Hello 你好` text at `(50, 200)`.
+  - Blue stroked rect outline at `(200, 50) — 200x150`.
+  - Closing the window exits cleanly.
+- Running under Xvfb + Mesa llvmpipe is acceptable for "no real GPU"
+  hosts; the chrome must still render correctly.
+
+## Where to record results
+
+Append the run date, distro/version, GL renderer (`glxinfo | grep
+OpenGL.renderer`), and output excerpt to this file (replacing the
+`Status: PENDING` line) and commit on `v0.8.0` with
+`docs(shell-native): record Linux manual GPU smoke`.
diff --git a/crates/openpencil-shell-native/notes/step-1a-macos-manual-smoke.md b/crates/openpencil-shell-native/notes/step-1a-macos-manual-smoke.md
new file mode 100644
index 000000000..7f4d94ea2
--- /dev/null
+++ b/crates/openpencil-shell-native/notes/step-1a-macos-manual-smoke.md
@@ -0,0 +1,42 @@
+# Step 1a macOS manual GPU smoke (per spec v19 §1.2 acceptance #1)
+
+**Status**: PASS — exercised on Apple Silicon during Phase C Task 4
+implementation (2026-05-05).
+
+The macOS path through `gpu_smoke` / `gpu_chrome_stub_composition`
+already runs in CI on `macos-latest` (`SharedSkiaContext::new_desktop`
+
+- raster + chrome+stub composition). This note captures the
+  maintainer-run `cargo run --example basic_window` smoke in addition
+  to the automated checks (spec v19 §1.2 acceptance #1).
+
+## Run on Apple Silicon (macos-latest, M-series)
+
+```bash
+cargo run -p openpencil-shell-native --example basic_window
+```
+
+### Expected window contents
+
+- 800x600 window titled `OpenPencil — basic_window (Step 1a §1.2 acceptance #1)`.
+- White background.
+- Red filled rect at `(50, 50) — 100x100` (chrome).
+- Black `Hello 你好` text at `(50, 200)` (chrome via Jian skia
+  textlayout).
+- Blue stroked rect outline at `(200, 50) — 200x150`.
+- Closing the window via Cmd-W / red button → process exits with
+  status 0 (no panic, no driver complaint).
+
+### Verified
+
+- Build: `cargo build --examples --workspace` succeeds clean.
+- Process: launches without stderr output, holds the window until
+  closed, no hangs on teardown.
+- `SharedSkiaContext::teardown` runs once via `exiting()` and is a
+  no-op on `Drop`.
+
+## When to update this file
+
+- After upgrading `vendor/jian` / `glutin` / `winit` / `skia-safe`.
+- After reworking `paint_chrome` to call new `NativeBackend` methods.
+- After macOS releases that change EAGL / Metal-translated bridging.
diff --git a/crates/openpencil-shell-native/notes/step-1a-windows-manual-smoke.md b/crates/openpencil-shell-native/notes/step-1a-windows-manual-smoke.md
new file mode 100644
index 000000000..a145b05be
--- /dev/null
+++ b/crates/openpencil-shell-native/notes/step-1a-windows-manual-smoke.md
@@ -0,0 +1,47 @@
+# Step 1a Windows manual GPU smoke (per spec v19 §8.1)
+
+**Status**: PENDING — to be filled by a maintainer running on a Windows
+desktop with a real GL driver.
+
+The standard GitHub Actions `windows-latest` runner has no GPU driver
+(`WINDOWS_GPU_DEFERRED_NO_RUNNER`), so spec v19 §8.1 requires the
+following sequence to be exercised by a human on real hardware before
+Step 1a can be declared "live on Windows".
+
+## Prerequisites
+
+- Windows 10/11 (x86_64 or aarch64) with a working OpenGL 3.3+ driver
+  (default factory drivers on most modern GPUs satisfy this).
+- Rust toolchain 1.85 (via `rustup toolchain install 1.85`).
+- Submodules checked out (`git submodule update --init --recursive`) so
+  `vendor/jian` is at `c4a794dc` (Step 1a freeze).
+
+## Required commands
+
+```pwsh
+cargo run -p openpencil-shell-native --example basic_window
+cargo test -p openpencil-shell-native --test gpu_smoke -- --include-ignored gpu_smoke
+cargo test -p openpencil-shell-native --test gpu_chrome_stub_composition -- --include-ignored gpu_chrome_stub_composition
+```
+
+## Expected outcomes
+
+- `basic_window` opens an 800x600 window showing:
+  - White background.
+  - Red filled rect at `(50, 50) — 100x100` (chrome).
+  - Black `Hello 你好` text at `(50, 200)` (chrome via Jian skia
+    textlayout).
+  - Blue stroked rect outline at `(200, 50) — 200x150`.
+  - Closing the window exits cleanly (no panic, no driver complaint
+    in the console).
+- `gpu_smoke` (`#[ignore]`'d on Windows by `WINDOWS_GPU_DEFERRED_NO_RUNNER`)
+  passes when run with `--include-ignored` on a real-GPU host.
+- `gpu_chrome_stub_composition` likewise passes — chrome pixel reads
+  back red even after `CanvasViewportStub::render_into` pollutes
+  `STENCIL_TEST` and `BlendFunc(ONE, ZERO)`.
+
+## Where to record results
+
+Append the run date, Windows build, GPU/driver, and output excerpt to
+this file (replacing the `Status: PENDING` line) and commit on
+`v0.8.0` with `docs(shell-native): record Windows manual GPU smoke`.
diff --git a/tools/check-jian-boundaries.sh b/tools/check-jian-boundaries.sh
new file mode 100755
index 000000000..6fbf88100
--- /dev/null
+++ b/tools/check-jian-boundaries.sh
@@ -0,0 +1,106 @@
+#!/usr/bin/env bash
+# Step 1a Phase C Task 4 / spec v19 §11 + §12.3 boundary invariants.
+#
+# Verifies the following Jian crate boundary invariants from outside the
+# Rust build system. Run from the repo root.
+#
+# Invariant 1 (§12.3): openpencil-app must NOT depend directly on any
+#   `jian-*` crate — Jian is a shell-native implementation detail; the
+#   app only sees OP's `RenderBackend` / `ShellEvent` facade.
+#
+# Invariant 2 (§11.1, §12.3): mobile targets (`aarch64-linux-android`,
+#   `aarch64-apple-ios`) must NOT pull `jian-host-desktop` or `jian-skia`
+#   into the dependency closure — those carry the desktop GL stack +
+#   `skia-safe` build.rs that fails on cross-compile.
+#
+# Invariant 3 (§11.1, §1.2): wasm32 builds of `openpencil-shell-web`
+#   must NOT pull `jian-host-desktop` or `jian-skia` (skia-safe build.rs
+#   fails on wasm32; Jian-core is wasm32-clean per P0.5 and is the only
+#   Jian crate allowed in the bundle).
+#
+# Invariant 4 (§1.2): `openpencil-shell-web` must NOT depend on
+#   `jian-host-desktop` at all — even as a non-default optional dep.
+#
+# Exit codes:
+#   0  — all invariants pass.
+#   1+ — one or more invariants fail; the failing crate names are
+#         echoed before the script exits.
+#
+# Dependencies: `cargo`, `jq` (for cargo metadata JSON parsing).
+set -euo pipefail
+
+if ! command -v jq >/dev/null 2>&1; then
+    echo "check-jian-boundaries.sh: \`jq\` is required but not installed." >&2
+    echo "  apt: sudo apt-get install -y jq" >&2
+    echo "  brew: brew install jq" >&2
+    exit 2
+fi
+
+# ── Invariant 1: openpencil-app has no direct jian-* dependency. ──────
+# `cargo metadata` returns a workspace-wide resolve graph; we filter
+# the `resolve.nodes[]` entry whose name matches `openpencil-app` and
+# inspect its direct `deps[]`. A direct dep on any `jian-*` crate
+# fails the invariant.
+metadata_full="$(cargo metadata --format-version 1)"
+forbidden_app="$(echo "$metadata_full" | jq -r '
+    [.packages[] | select(.name == "openpencil-app") | .id] as $app_ids
+    | .resolve.nodes[]
+    | select(.id as $id | $app_ids | index($id))
+    | .deps[].name
+' | grep -E '^jian-' || true)"
+if [ -n "$forbidden_app" ]; then
+    echo "INVARIANT 1 FAILED: openpencil-app directly depends on jian-* crate(s):" >&2
+    echo "$forbidden_app" >&2
+    exit 1
+fi
+
+# ── Invariant 2: mobile targets don't pull jian-host-desktop / jian-skia. ──
+# We use `cargo tree` (which honours `--target` cfg-gates) and inspect
+# the dependency closure of `openpencil-shell-native` — only the deps
+# that actually compile under the mobile target are listed.
+for target in aarch64-linux-android aarch64-apple-ios; do
+    tree_mobile="$(cargo tree -p openpencil-shell-native \
+        --target "$target" \
+        --prefix none \
+        --edges normal,build 2>/dev/null || true)"
+    forbidden_mobile="$(echo "$tree_mobile" \
+        | grep -oE '\bjian-(host-desktop|skia)\b' \
+        | sort -u || true)"
+    if [ -n "$forbidden_mobile" ]; then
+        echo "INVARIANT 2 FAILED ($target): forbidden Jian crates in closure:" >&2
+        echo "$forbidden_mobile" >&2
+        exit 1
+    fi
+done
+
+# ── Invariant 3: wasm32 has no jian-host-desktop / jian-skia. ──────────
+# `jian-core` IS allowed (P0.5 wasm32-clean).
+tree_wasm="$(cargo tree -p openpencil-shell-web \
+    --target wasm32-unknown-unknown \
+    --prefix none \
+    --edges normal,build 2>/dev/null || true)"
+forbidden_wasm="$(echo "$tree_wasm" \
+    | grep -oE '\bjian-(host-desktop|skia)\b' \
+    | sort -u || true)"
+if [ -n "$forbidden_wasm" ]; then
+    echo "INVARIANT 3 FAILED: wasm32 openpencil-shell-web pulls forbidden Jian crates:" >&2
+    echo "$forbidden_wasm" >&2
+    exit 1
+fi
+
+# ── Invariant 4: openpencil-shell-web has no jian-host-desktop dep. ───
+# Distinct from invariant 3 (which checks the resolved closure on the
+# wasm32 target): this checks the manifest itself across all targets.
+# `cargo tree --all-targets` would include dev-deps; we explicitly
+# filter `--edges normal,build` for the manifest-level invariant.
+shell_web_deps="$(cargo tree -p openpencil-shell-web \
+    --prefix none \
+    --edges normal,build 2>/dev/null \
+    | grep -E '\bjian-host-desktop\b' || true)"
+if [ -n "$shell_web_deps" ]; then
+    echo "INVARIANT 4 FAILED: openpencil-shell-web depends on jian-host-desktop:" >&2
+    echo "$shell_web_deps" >&2
+    exit 1
+fi
+
+echo "check-jian-boundaries.sh: all 4 Jian boundary invariants pass."