refactor(shell): single canonical MIN_RAIL_WIDTH in shell-core

Codex Step 3 R1 BLOCK: `MIN_RAIL_WIDTH: f32 = 80.0` was defined
twice — once in `crates/openpencil-shell-web/src/widget_host.rs`
and once in `crates/openpencil-shell-native/src/widget_host.rs`.
Each had a comment claiming "mirrors the other"; nothing
enforced agreement. A future drift on one side would silently
break cross-platform layout parity.

Move to a single canonical `pub const MIN_RAIL_WIDTH: f32 = 80.0`
in `crates/openpencil-shell-core/src/widgets/mod.rs`. Both hosts
import it via the existing `widgets::*` use list.

Verification:
- `cargo build -p openpencil-shell-native --example
  inspector_window` — green
- `cargo build -p openpencil-shell-web --target
  wasm32-unknown-unknown --features skia --release` — green
- `cargo test -p openpencil-shell-core --lib` — 39 tests passing
- grep confirms one definition + two imports + 4 use sites

crates/openpencil-shell-core/src/widgets/mod.rs

@@ -17,6 +17,15 @@
 
 use crate::{Point2D, Rect, RenderBackend};
 
+/// Minimum width (in CSS / physical px) below which the editor-UI
+/// host paints the Toolbar only and skips the LayerPanel /
+/// CanvasViewport / PropertyPanel rails. Single canonical
+/// definition consumed by both `openpencil-shell-web::WidgetHost`
+/// and `openpencil-shell-native::WidgetHostNative` so they stay in
+/// lock-step (codex Step 3 R1 BLOCK fix — was duplicated as
+/// `const MIN_RAIL_WIDTH` in each host).
+pub const MIN_RAIL_WIDTH: f32 = 80.0;
+
 // Phase B primitives.
 pub mod dropdown;
 pub mod prop_row;

crates/openpencil-shell-native/src/widget_host.rs

@@ -52,16 +52,10 @@
 use crate::backend::NativeBackend;
 use openpencil_shell_core::document::Document;
 use openpencil_shell_core::widgets::{
-    CanvasViewport, LayerPanel, LayoutCx, PaintCx, PropertyPanel, Toolbar, Widget,
+    CanvasViewport, LayerPanel, LayoutCx, PaintCx, PropertyPanel, Toolbar, Widget, MIN_RAIL_WIDTH,
 };
 use openpencil_shell_core::{Color, Point2D, Rect, RenderBackend, TextLayout};
 
-/// Below this viewport width (in physical px) we paint the Toolbar
-/// only and skip both rails. Mirrors shell-web's `MIN_RAIL_WIDTH`
-/// constant so cross-platform behaviour at narrow viewports stays
-/// in sync. Codex Step 2 R1 CONCERN-3.
-const MIN_RAIL_WIDTH: f32 = 80.0;
-
 /// Frame-scoped `RenderBackend` adapter over `NativeBackend` +
 /// `&Canvas`. Lifetime-bound to the `SharedSkiaContext::with_frame`
 /// closure body so widget code never sees the canvas borrow directly.

crates/openpencil-shell-web/src/widget_host.rs

@@ -24,17 +24,10 @@
 use crate::backend::WebBackend;
 use openpencil_shell_core::document::Document;
 use openpencil_shell_core::widgets::{
-    CanvasViewport, LayerPanel, LayoutCx, PaintCx, PropertyPanel, Toolbar, Widget,
+    CanvasViewport, LayerPanel, LayoutCx, PaintCx, PropertyPanel, Toolbar, Widget, MIN_RAIL_WIDTH,
 };
 use openpencil_shell_core::{Point2D, Rect, RenderBackend};
 
-/// Below this viewport width (in CSS px) we paint the Toolbar
-/// only and skip both rails — there's not enough horizontal
-/// space for a usable LayerPanel + PropertyPanel split. Codex
-/// Step 2 R1 CONCERN-3: clamp `rail_w` to non-negative AND skip
-/// when too small to be useful.
-const MIN_RAIL_WIDTH: f32 = 80.0;
-
 /// The Step 3 editor-UI host. Owns the document model + toolbar
 /// tool selection; per-frame builds LayerPanel / PropertyPanel /
 /// CanvasViewport from the document. Aux Dropdown / TextInput