End-to-end testing on macOS + BlueBubbles + ElevenLabs walked through three CAF flavors before landing on the format Apple's Messages.app actually emits when a user records a native iMessage voice memo:
- PCM int16 @ 44.1 kHz CAF: BlueBubbles' internal `afconvert -f m4af -d aac` conversion fails; the original CAF reaches iMessage but renders with 0 s duration.
- AAC @ 22.05 kHz mono CAF: BlueBubbles' conversion succeeds and the server silently downgrades the delivery, sending the converted MP3 as a generic audio attachment.
- **Opus @ 24 kHz mono CAF**: byte-identical to the descriptor block Apple's Messages.app produces; BlueBubbles passes it through unchanged and iMessage renders a native voice-memo bubble with proper duration and waveform UI.
Adds an opt-in `tts.voice.preferAudioFileFormat` channel capability and a macOS `afconvert`-backed pre-transcode in the speech-core pipeline. BlueBubbles declares `preferAudioFileFormat: "caf"`. Other channels are unaffected. Falls back to the original buffer when the host platform, the source/target pair, or the transcoder process can't produce the preferred container — so non-Darwin hosts and unsupported provider combinations are unchanged.
Also adds a `caff` magic-byte sniff in `src/media/mime.ts` so the auto-reply host-local-media validator (which uses `file-type` and didn't recognize CAF natively) accepts the buffer instead of dropping it as "⚠️ Media failed."
Fixes#72506.
Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
