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/session/session.cpp

@@ -45,6 +45,7 @@ std::vector<Stream> copy_streams(
         s.expected_packet_loss = pb.audio().expected_packet_loss();
         s.dtx                  = pb.audio().dtx();
         s.complexity           = pb.audio().complexity();
+        s.dred                 = pb.audio().dred();
         out.push_back(std::move(s));
     }
     return out;

core/src/session/session.h

@@ -45,6 +45,7 @@ struct Stream {
     uint32_t    expected_packet_loss{0};
     bool        dtx{false};
     uint32_t    complexity{0};
+    bool        dred{false};
 };
 
 struct User {