The first streamWatchReceiver only did dave.session.decrypt(msg.subarray(12))
which skipped the outer legacy-AES layer Discord wraps around the DAVE payload
on every RTC packet (encrypt = dave.encrypt then aead_aes256_gcm with RTP
header as AAD). Port @discordjs/voice VoiceReceiver.decrypt/parsePacket
faithfully (header strip incl CSRC+extension+padding, AES-GCM auth tag, then
DAVE MediaType.VIDEO). Without this the .h264 would be garbage.