fix: address code review feedback (PR #13, #15)

[ZhaoChaoqun]

Summary

• AudioEngineManager: Add thread-safety doc comments to onSamples and onAudioLevel callbacks, clarifying they must be set before start() and not mutated while the engine is running
• ASREngineFactory: Remove unused termNormalizer field from Result struct (the pipeline sets it directly, caller never reads it)
• ModelDownloader: Remove dead destDir assignment and clarify why the second parameter of extractionHandler is unused

Test plan

• Build succeeds (xcodebuild -scheme Typeless -configuration Debug)
• Verify no runtime regressions in ASR model loading
• Verify no runtime regressions in model downloading/extraction

🤖 Generated with Claude Code

[claude]

Review Summary

Clean, well-scoped PR. All three changes verified:

1. ASREngineFactory.Result — termNormalizer removal is safe. The only caller (RecordingManager.swift:208) accesses result.engine only; the normalizer is set directly on the pipeline at factory line 58.

2. AudioEngineManager — Doc comments accurately describe the threading contract. Both onSamples and onAudioLevel are indeed set before start() in current usage. Note: there's no runtime enforcement (no locks/actors) — the contract is convention-based, which the docs now make explicit.

3. ModelDownloader — Dead destDir assignment correctly removed (both ternary branches were identical). Comment about unused second parameter is accurate — the handler closure (SherpaOnnxManager.swift:134) captures modelsDirectory directly and ignores the parameter with _.

No issues found.