_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>
jpmschweitzerandClaude Opus 4.7
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