docs(readme): refresh Step 1a entry to spec v19.5 (drop stale ShellEvent/JianPointerMapper reference)

Codex stop-hook caught README:505 still mentioned the v19.4-removed
ShellEvent + JianPointerMapper translation layer. Updated to reflect:
- shell-core re-exports jian_core::gesture::* directly (events consistent
  with Jian per v19.4)
- spec v19.5 FROZEN
- full multi-OS CI matrix (now incl Windows aarch64 cross, macOS x86_64
  cross from Apple Silicon, Linux aarch64 cross)
- OP visual model + chrome Rust-only constraint deferred to Step 1c+

README.md

@@ -502,7 +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`.
+- [x] Rust shell — Step 1a (G1 shared Skia context) on `v0.8.0`: `SharedSkiaContext` + `NativeBackend` (Jian-`DrawOp`-backed) + shell-core re-exports `jian_core::gesture::*` (events stay consistent with Jian — no OP-side translation layer per v19.4) + `basic_window` demo, with the multi-platform CI matrix green (macOS aarch64 + x86_64, Linux x86_64 + aarch64 cross, Windows x86_64 + aarch64 cross, iOS aarch64 + sim, Android aarch64 + x86_64, wasm32). Spec `v19.5` FROZEN; `vendor/jian` pinned at `c4a794dc`. OP visual model (single-page + infinite canvas canonical / multi-page also supported / no routing / Figma import auto-converts) and OP chrome (toolbars / panels — Rust-only) deferred to Step 1c+.
 - [ ] Collaborative editing
 - [ ] Plugin system