feat: DRED (Deep REDundancy) per-channel toggle

Adds Opus 1.6 DRED support end-to-end: encoder embeds 20 ms of ML
redundancy in every packet when enabled; decoder recovers lost frames
from the next buffered packet's DRED extension rather than falling back
to PLC comfort noise.

Protocol: bool dred = 11 added to AudioConfig (backward-compatible,
defaults false). C ABI: int dred added to vc_audio_config. Encoder:
OPUS_SET_DRED_DURATION(2) when dred=true. Decoder: OpusDREDDecoder +
per-stream OpusDRED scratch pre-allocated off the RT thread;
JitterBuffer::try_copy_front_payload peeks at the next packet without
popping on every PLC step; opus_decoder_dred_decode reconstructs the
lost frame if DRED data is present, otherwise falls back to PLC.

New test: test_dred_toggle (22/22 ctest green).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

core/proto/voicecat.proto

@@ -87,6 +87,7 @@ message AudioConfig {
   uint32 expected_packet_loss = 8;  // %
   bool   dtx = 9;
   uint32 complexity = 10;           // 0..10
+  bool   dred = 11;                 // Deep REDundancy (Opus 1.6); off by default
 }
 
 message StreamInfo {