[Bug]: macOS — a denied system-audio permission silently records digital silence with no warning (2.0.0-rc.1) #877

Describe the bug

On macOS 26.5, when macOS denies OpenScreen's system-audio capture request at take start, the app silently records a digital-silence audio track and surfaces no warning anywhere in the recording flow. The user only discovers the recording has no sound at playback.

Observed concretely:

• Take recorded with System audio On (Apple's screen picker flow, v2.0.0-rc.1).
• During the take, a 440 Hz tone was played through the speakers for 14 s (afplay).
• At take start macOS denied the SystemAudioRecordingOnly request (it was auto-denied while another TCC prompt already held the screen).
• The saved take carries an AAC 48 kHz stereo track that is pure digital silence: ebur128 integrated ≈ −70 LUFS, sample peak −inf dBFS — the 440 Hz tone is absent.
• No toast, no badge, no indicator on the HUD or in the editor. The only signal is the permissions window's System-audio row flipping to the refused state ("Open Settings" / "If you said no, turn on OpenScreen under 'System Audio Recording Only'…") — which the user has no reason to re-open after a take that appeared to record normally.

Expected behavior

Either:

1. system audio is captured (permission granted), or
2. the denial is surfaced at take time — a toast/banner on the HUD ("macOS refused system-audio capture — this recording will have no sound") and ideally the editor opening with an audible warning on the asset, and/or no silent audio track muxed at all.

The permissions-window row state is correct behavior; the gap is that a take recorded under denial is indistinguishable from a normal one until played back.

To Reproduce

1. Deny OpenScreen "System Audio Recording Only" in System Settings (or have another TCC prompt up at take start so the request is auto-denied).
2. Record a screen take with System audio On while playing any audio.
3. Stop and inspect the recording: video fine, audio track present but silent.

Environment

• Build: CI artifact openscreen-mac-arm64, build run 36336647912, 2.0.0-rc.1 (dmg)
• OS: macOS 26.5 (25F71), Apple M1 Mac mini
• System-audio TCC state: denied (the row's refused state was confirmed)

Additional context

Found during the v2.0.0-rc.1 release-candidate manual pass. This also blocks verifying #740's "system audio really recorded, checked in the exported file" on this machine without an owner-granted permission. Related UX hazard: a user who records a meeting this way loses the audio permanently.