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/include/voicecat.h

@@ -221,6 +221,7 @@ typedef struct vc_audio_config {
     uint32_t expected_packet_loss; /* % 0..100 */
     int      dtx;                  /* bool */
     uint32_t complexity;           /* 0..10 */
+    int      dred;                 /* bool — Deep REDundancy (Opus 1.6), off by default */
 } vc_audio_config;
 
 /* M5: permission bitset (mirrors protocol Permissions). */