feat(watchdog): dedicated-isolate heartbeat + resource sampler (T-435)

A main-isolate Timer would freeze WITH the main isolate and tell us nothing,
so the watchdog runs in its own isolate: it fsyncs a heartbeat every ~500ms
(so the last on-disk heartbeat bounds a freeze to ~500ms) and every ~2s samples
this process's thread / handle-or-fd / child-host / RSS counts. A monotonically
climbing child or thread count is the leak signature the soak couldn't
reproduce on CI but a real freeze would show. Output is JSON-lines in
clide-watchdog.log, bounded by the same truncate-on-cap scheme as the crumb
files.

- watchdog.dart (Flutter-free, tested): ResourceSample, ResourceSampler
  (forPlatform), PosixResourceSampler (/proc/self: Threads, fd count, task
  children, ProcessInfo.currentRss), WatchdogFile (bounded fsynced JSON-lines),
  runWatchdog (the loop, bounded by maxTicks for tests), watchdogEntry (the
  sendable Isolate.spawn entry).
- watchdog_windows.dart (coverage:ignore — Win32 FFI, validated only at runtime
  on Windows): one CreateToolhelp32Snapshot for thread + conhost/OpenConsole
  child count, GetProcessHandleCount, ProcessInfo.currentRss. Exhaustively
  defensive: any failure yields a -1 field, snapshot handle always closed, never
  throws.
- main.dart: spawn the watchdog at boot (desktop only), non-fatal. Per-line
  fsync means the OS reaping the isolate at exit loses nothing.

Tests: ResourceSample.toJson, Posix sampler against real /proc, WatchdogFile
(JSON shape, cap, disabled), runWatchdog (immediate baseline tick). Coverage
gate 95.08%.

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

lib/kernel/kernel.dart

@@ -18,6 +18,7 @@ export 'src/events/types.dart';
 export 'src/ipc/client.dart';
 export 'src/log.dart';
 export 'src/file_log_sink.dart';
+export 'src/watchdog.dart';
 export 'src/settings.dart';
 export 'src/facade.dart';
 export 'src/clipboard.dart';