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/src/codec/opus_codec.h

@@ -34,6 +34,7 @@ struct OpusParams {
     uint32_t        complexity           = 10;
     uint32_t        expected_packet_loss = 0;   // % 0..100
     OpusApplication application           = OpusApplication::Voip;
+    bool            dred                 = false;
 };
 
 // Returns frame_samples for a given sample_rate + frame_ms.
@@ -95,6 +96,14 @@ class OpusDecoder {
     int decode(const uint8_t* opus_data, int len, int16_t* out_pcm, int max_samples,
                bool fec = false);
 
+    // Decode a lost frame using pre-parsed DRED state from the next received packet.
+    // dred_offset=0 means the frame immediately before the next packet.
+    // Returns frame_samples on success, -1 if DRED unavailable or decode failed.
+#ifdef VOICECAT_HAS_OPUS
+    int decode_dred(::OpusDRED* dred, int32_t dred_offset, int16_t* out_pcm, int max_samples);
+    ::OpusDecoder* raw() const { return dec_; }
+#endif
+
     void destroy();
 
     bool        valid()        const { return dec_ != nullptr; }