feat(audio): share meeting mic with dictation (resurrected branch — needs review)

[r3dbars]

Resurrected branch — opened as a draft by branch-triage. Was pushed 2026-06-06 with no PR and would otherwise have been pruned. Needs human review before it's ready.

What it is

Lets meeting capture and dictation share the same microphone instead of blocking each other. Earlier the branch experimented with hard exclusivity ("Block overlapping meeting and dictation capture"), reverted it, and landed on a shared-mic relay instead.

Adds (all novel — none of this exists on origin/main):

• MeetingSessionController.canShareMicWithDictation + installSharedDictationMicRelay() / clearSharedDictationMicRelay()
• STTRouter / ParakeetEngine: startRecordingFromSharedMeetingMic(), handoffActiveRecordingToSharedMeetingMic(), appendSharedMeetingMicBuffer(), isRecordingFromSharedMeetingMic, resume-after-shared-mic handling
• MeetingCaptureBridge shared dictation-mic handler plumbing
• DictationSessionController integration + RepoCommandContractTests coverage

7 files, +298/-7, including tests.

Status / caveats

• 3 commits, branch is ~462 commits behind origin/main but merges cleanly (0 conflicts) against current main as of triage.
• grep canShareMicWithDictation origin/main → 0 hits, so this is not superseded by anything merged.
• Touches real-time audio paths (ParakeetEngine, CoreAudio capture relay) — needs careful review against the "no I/O in CoreAudio callbacks" rule and the recent parakeet tap-teardown crash fix (PR fix(audio): stop dictation engine before removing input tap (isSink || tap != nullptr crash) #1135) before merge.

Draft intentionally. Close if the shared-mic direction has been abandoned.