fix(recording): preserve queued audio and prepare 1.2.1

[rteoo]

Summary

Stopping just after the native capture helper exits raised before the controller drained buffered PCM, leaving queued audio out of the saved recording. Make Stop idempotent for an exited helper, including exit during the pipe write, so queued audio is journaled while EOF, overflow and nonzero exits remain visible.

The Windows preparation helper also accepted inconsistent source-header/runtime metadata. Validate runtime, installer and bundle workflow versions/channels before tests and recheck them after tests/builds. Synchronize the upcoming patch version to 1.2.1.

Testing

• Reproduced the stop race with a real subprocess fixture: buffered PCM was absent from the saved recording before the fix. Reproduced acceptance of mismatched runtime version metadata.
• Complete Windows suite: 1,190 tests, 10 skips, no failures. Focused capture/controller/packaging suite: 100 tests passed.
• Windows release helper: 15 scenarios passed, including metadata drift during tests.
• Ruff and Git formatting checks passed; actual candidate metadata validated as 1.2.1 stable.
• No standalone secret scanner available; manual staged-content/outgoing-commit privacy review completed. Generated logs and reports are excluded from commits.

Risks

Local MP3 integration was skipped because the clean PyAV runtime is absent. Final installed-package Teams recording, minimized feedback and final signing/notarization remain release gates. This PR does not publish a release.