Recording (Windows): end the audio track where the video ends

[EtienneLescot]

Part of #920 (rank 26). Effort S, risk low.

Problem

• On Windows the audio track runs 0.44 to 0.58 s past the video, on three takes of 13 to 113 s (measured with ffprobe). The gap is constant, so it is a tail, not drift.
• At stop, the helper quiesces WGC and stops the microphone, loopback, device watcher, keep-alive and webcam, each within its own time budget, and only then stops the audio mixer (main.cpp stop sequence). The mixer is clock-driven, so it keeps writing chunks, silence included, for that whole half second.

Proposal

• Freeze the mixer at the instant the stop is requested (the pause path: it writes what the cushion holds up to that instant, then emits nothing more), so the audio ends where the video does.

Verify

• A real take: ffprobe -show_entries stream=codec_type,duration reads audio and video within one audio chunk (10 ms) of each other.

[github-actions]

Fixed by #946 and merged into main.

This change is assigned to the Next Release release milestone and is not necessarily available in the latest downloadable desktop release yet. It is currently marked as status: pending release until a packaged release containing it is published.

[EtienneLescot]

Moved into v2.0.0 as part of the pre-release cut.