fix(windows): end the audio track where the video ends

[EtienneLescot]

Fixes #942. Part of #920.

Cause

• Every Windows take carried 0.44 to 0.58 s of audio past its video (three takes of 13 to 113 s; a constant gap, so 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 budget, and only then stops the mixer. The mixer is clock-driven: it kept writing chunks, silence included, through that half second.

Change

• One call: the mixer is paused on the stop request, the same request the video writer stops on.
• The pause writes what the 100 ms cushion holds up to that instant (fix(windows): stop the audio mixer from punching holes in a continuous voice #916), then nothing. A pause already in place stays where it is.

Verified

• New mixer-track-ends-at-the-pause-before-a-stop: a pause followed 300 ms later by a stop ends the track at the pause (410 ms for a pause at 418 ms, the 10 ms chunk grid). Without the pause, the stop would carry it 300 ms further.
• Full native build, all native tests pass (121 in audio_sample_utils_test).

Pending

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

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📒 Files selected for processing (2)

• electron/native/wgc-capture/src/audio_sample_utils_test.cpp
• electron/native/wgc-capture/src/main.cpp

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