[Bug]: macOS: system audio is silent in a window recording when the sound comes from another app #1216

Platform: macOS 26.5 (M1), CI build of main @ 0bed36852 (run 38581408372). Found during the e2e pass logged in #1212.

Repro

1. Through the stdio MCP server: startRecording with window: "<a Finder window>", systemAudio: true.
2. Play sound from another process (afplay speech.m4a, on the Mac's speakers) during the take, then stopRecording.
3. ffmpeg -i <take>.mp4 -af volumedetect -f null -.

Expected: the system audio is in the take, as on Windows (loopback of the whole mix) and as "Also record the computer's sound" suggests.

Actual: the audio track exists but is digital silence: mean_volume: -91.0 dB. The same sound recorded with display: 0 reads -10.5 dB.

Cause: ScreenCaptureRecorder.swift builds a window take's filter with SCContentFilter(desktopIndependentWindow:). ScreenCaptureKit then captures audio only from the window's own application, so a Finder window records Finder's silence. A take of a browser window would carry the browser's sound only. The HUD's window source uses the same helper path, so this is likely not specific to MCP.

Direction: for system audio on a window take, use an audio source that is not scoped to that app. Two options: a second display-scoped SCStream for audio only, or the Core Audio process tap the picker path already uses. Alternatively, document that a window take records only that app's sound and say so in the HUD and in startRecording's description.