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>

lib/kernel/src/watchdog.dart

@@ -25,7 +25,9 @@ library;
 import 'dart:convert';
 import 'dart:io';
 
-import 'watchdog_windows.dart';
+// Web fence (T-438, D-100): the FFI-backed Windows sampler is reachable only
+// when `dart.library.ffi` is available; the web build gets an all-`-1` stub.
+import 'watchdog_windows_stub.dart' if (dart.library.ffi) 'watchdog_windows.dart';
 
 /// One resource sample of the current process. A field of `-1` means "not
 /// available on this platform or the probe failed" — never an error.