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/test_app.dart

@@ -25,10 +25,11 @@ import 'builtin/files/files.dart';
 import 'builtin/git/git.dart';
 import 'builtin/terminal/terminal.dart';
 import 'extension/extension.dart' show ClideExtension;
-import 'dart:ffi' as ffi;
-import 'package:ffi/ffi.dart' as pkg_ffi;
 import 'kernel/kernel.dart';
-import 'src/pty/ffi/libc.dart' as libc;
+
+// Web fence (T-438, D-100): the FFI fd-inheritance probe is desktop-only; the
+// web build gets a no-op stub so dart:ffi / package:ffi / libc stay out.
+import 'fd_check_stub.dart' if (dart.library.ffi) 'fd_check_io.dart';
 import 'src/daemon/pane_commands.dart';
 import 'src/ipc/envelope.dart';
 import 'src/ipc/paths.dart' show logDirectory;
@@ -375,20 +376,9 @@ class _ClideTestAppState extends State<ClideTestApp> {
       return output.isNotEmpty ? 'got ${output.length} chars' : 'no output (0 chars)';
     });
 
-    // Test: does Dart's Process.start inherit socket fds on macOS?
-    await _testAsync('fd inheritance check', () async {
-      final sv = pkg_ffi.calloc<ffi.Int32>(2);
-      libc.socketpair(1, 1, 0, sv); // AF_UNIX, SOCK_STREAM
-      final parent = sv[0];
-      final child = sv[1];
-      pkg_ffi.calloc.free(sv);
-      final proc = await Process.start('/tmp/checkfd', [], environment: {...Platform.environment, 'PTYC_SOCK_FD': '$child'});
-      final stderr = await proc.stderr.transform(utf8.decoder).join();
-      final exit = await proc.exitCode;
-      libc.close(parent);
-      libc.close(child);
-      return 'exit=$exit stderr=${stderr.trim()}';
-    });
+    // Test: does Dart's Process.start inherit socket fds on macOS? (T-438: the
+    // FFI body lives in fd_check_io.dart so the web build can stub it out.)
+    await _testAsync('fd inheritance check', fdInheritanceCheck);
 
     _say('');
   }