streamWatchReceiver: DAVE decrypt for screen-share video packets was
returning null every time (VIDEO-PKT diag showed packets arriving but
no 'Video segment opened'). Three possible causes:
1. No VIDEO decryptor in MLS group (audio-only handshake)
2. GoLive stream tags video packets as AUDIO
3. Screen-share payloads are unencrypted above the AES layer
Fix: try MediaType.VIDEO first, then MediaType.AUDIO, then passthrough
(legacy-decrypted payload as-is). This covers all three modes without
breaking audio recording.