fix SchedulerService isolate-spawn race (T-106)

_startTicker fired Isolate.spawn(...).then((iso) => _isolate = iso)
and returned. If _stopTicker landed before the spawn future resolved,
_isolate was still null at kill time and the just-spawned isolate
(with its Timer.periodic) leaked forever.

Track the spawn as _isolateReady and have _stopTicker await it before
killing. Same shape as the NativePty fix from T-96.

dispose() is now async; the single caller in facade.dart already
sat inside an async dispose chain and just needed the await.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

lib/kernel/src/facade.dart

@@ -238,7 +238,7 @@ class KernelServices {
     focus.dispose();
     project.dispose();
     extensions.dispose();
-    scheduler.dispose();
+    await scheduler.dispose();
     await log.dispose();
     messages.dispose();
     await events.dispose();