[bug-hunter] Guard meeting capture against stale stop completions

[r3dbars]

Bug

Audio.stop() advances the capture generation synchronously but still emits its completion callback later with the stopped session's files. MeetingCaptureBridge previously matched that untagged callback to whichever stop continuation was current. If a timed-out meeting A was followed by meeting B, A's late callback could resolve B with A's files; starting B also discarded A's late-recovery callback.

Fix

• Emit a generation-tagged completion callback from Core while preserving the legacy callback for other embedders.
• Route bridge completions through an explicit expected/current/stale policy.
• Retain timed-out completion handlers keyed by generation, so A's late repair remains owned by A while B resolves independently.
• Add deterministic policy coverage for normal stop, late timeout, unexpected current completion, stale completion, and A-late/B-expected ordering.

Evidence and scope

• Confirmed against fresh origin/main.
• Independent adversarial review: GREEN. Reviewer confirmed the existing Core generation-race test demonstrates the old callback interleaving and that this is distinct from Repair speech route recovery on current main #1569 (speech-route recovery) and [bug-hunter] Retain active import recovery journal #1572 (import recovery journal).
• No raw transcript, audio, customer, or telemetry payloads are included.

Verification

• bash run-tests.sh: 13,052 passed, 0 failed.
• Focused bash run-tests.sh --filter testMeetingCaptureCompletionPolicy: 6 passed, 0 failed.
• git diff --check: passed.
• bash build-deps.sh --force: compiled 261 dependency targets, then environment-blocked while archiving libExternalDeps.a with errno=28 (No space left on device).
• bash build.sh --no-open: blocked because the dependency archive was not produced.
• bash run-integration-smoke.sh: blocked because dependencies were unavailable.
• swift test: blocked by the same missing dependency setup (no such module: FluidAudio).
• Manual real-Mac audio, permission, sleep/wake, Bluetooth, and crash/force-quit proof: UNKNOWN/not performed in this run.