feat(web): fence dart:ffi behind web stubs so the WASM build compiles (T-438, D-100)

`flutter build web --wasm` had been broken since the tree-sitter/PTY dart:ffi
pivot. Per D-100 (resolving Q-50: keep the web "happy accident" alive), every
native binding now sits behind a `dart.library.ffi` conditional import with a
graceful web stub. Desktop builds are unchanged — no fidelity loss; the web
target degrades (no terminal, native git, or syntax highlighting).

Discriminator is `dart.library.ffi`, not `dart.library.io` — dart2wasm provides
dart:io, so FFI is the only blocker.

Fences:
- PTY: pty_session → pty_backend_io / pty_backend_web (stub throws).
- tree-sitter: pure types → syntax_result.dart; tree_sitter_service is now a
  facade over _ffi/_stub; tree_sitter_boot_io/stub fences TreeSitterLib.init().
- watchdog: watchdog_windows_stub (all -1 sampler).
- claude ABI probe: native_abi_io/stub (was `dart:ffi show Abi`).
- testmode fd-check: fd_check_io/stub.

Also dart2js-safe: the 64-bit FNV literals in session_naming.dart + paths.dart
(the dual JS fallback rejected them) — split into 32-bit halves, dropped a
no-op 64-bit mask. Desktop/wasm hash values unchanged.

CI: added a `web-wasm` job (flutter build web --wasm) so the fence can't rot.
Two FFI-constructing tree-sitter tests import _ffi.dart directly (the analyzer
resolves the conditional facade to the stub branch).

Verified: `flutter build web --wasm` → built; `flutter analyze` clean;
`make test` green. Full Playwright e2e harness wiring is the tracked follow-on.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.github/workflows/test.yml

@@ -114,13 +114,21 @@ jobs:
           path: ${{ github.workspace }}/clide-logs
           if-no-files-found: ignore
 
-  # The web-WASM Playwright job is withheld: `flutter build web --wasm` cannot
-  # compile the tree since the tree-sitter/PTY dart:ffi pivot (dart:ffi is
-  # unavailable on the wasm target). Whether the web target gets conditional-
-  # import fences or is dropped is an open question — see Q-50 in
-  # governance/questions/architecture.md. Re-add the job (setup-node, npm
-  # install + playwright install in tools/ui, `make test-e2e`) when Q-50
-  # resolves toward keeping it.
+  web-wasm:
+    name: web build (wasm compile gate)
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: subosito/flutter-action@v2
+        with: { channel: stable, cache: true }
+      - run: flutter pub get
+      # Compile gate for the dart:ffi web fence (D-100 / T-438, resolving Q-50):
+      # every native binding lives behind a `dart.library.ffi` conditional import
+      # with a web stub. If a new one lands without its stub, this fails — the
+      # fence can't silently rot. Build-only for now; the full web-WASM Playwright
+      # e2e (`make test-e2e`: setup-node + playwright install in tools/ui +
+      # serve) is the follow-on once the harness is wired back up.
+      - run: flutter build web --wasm
 
   docs:
     name: dart doc (lib API)