Retire the from-scratch wasm32 skia raster web backend and the vendored
skia-safe fork now that CanvasKit is the sole web renderer.
- remove gl-webgl-shim + the `webgl` feature + backend/webgl.rs
- absorb chat streaming, live-sync, codegen, icon search, Figma/file IO,
and system fonts into the `canvaskit` build via a backend-agnostic
`RepaintContext` trait; wire them into `mount_ck` (chat send, image
paste routing, window resize)
- delete the skia raster modules (backend/, a11y, shell_drop/resize,
ime_target, boolean_ops, caret_pump, repaint_scheduler) + the skia
`Inner` mount; op-host-web is now `web` (stub) + `canvaskit` only
- drop [patch.crates-io]; delete vendor/skia-safe-op + crates/wasm-libc-shim
so skia-safe reverts to upstream crates.io 0.97.x (native unchanged,
byte-identical; upstream has no wasm32-unknown-unknown target)
- switch the bundle gate (check-wasm-bundle.sh) to --features canvaskit
Also float new shapes above the frame (z-order), inset the shape-picker
selected-row highlight, and align the web file-menu hover without the
traffic-light inset.
Builds clean: canvaskit + web-stub (wasm32) + native desktop; bundle gate
passes (0 env.*, 4.5 MiB gzip).
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