feat(capture): add durable microphone sessions

[douglasjarquin]

Closes #4

Summary

• Add a file-backed SessionStore with one Application Support directory per dictation.
• Atomically persist session.json before capture and model audio.caf, raw.txt, and clean.txt paths.
• Add AudioRecorder over AVAudioEngine input-node tap with mono linear PCM CAF capture, minimal buffer forwarding, and teardown on every terminal path.
• Integrate start, stop, cancel, interruption, failure, timing, recovery, playback, and reveal behavior through DictationCoordinator.
• Add focused contracts for atomic metadata, recovery, legal lifecycle, buffer forwarding, path safety, retention, playback, and 100 coordinator cycles.

Verification

• swift run oigo-issue4-contract-tests
• swift run oigo-issue3-contract-tests
• swift run oigo-spike-contract-tests
• swift build --product Oigo
• plutil -lint Oigo.xcodeproj/project.pbxproj
• plutil -lint Oigo/Info.plist
• git diff --check

The available host is Command Line Tools only.
Bundled-app microphone/TCC capture, force-quit recovery, 30-minute live recording, Instruments memory measurement, and native AVAudioEngine 100-cycle evidence remain explicitly unverified.