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>

tests/CMakeLists.txt

@@ -174,4 +174,13 @@ if(VOICECAT_USE_VCPKG_DEPS)
     target_include_directories(test_reaper_timeout PRIVATE ${VC_TEST_INTERNAL_INCLUDES})
     add_test(NAME reaper_timeout COMMAND test_reaper_timeout)
     set_tests_properties(reaper_timeout PROPERTIES TIMEOUT 30)
+
+    # DRED toggle: per-channel dred flag round-trips through protocol; encoder initialises
+    # with DRED; PCM injection through DRED-enabled encode path runs without crash.
+    add_executable(test_dred_toggle test_dred_toggle.cpp)
+    target_link_libraries(test_dred_toggle PRIVATE voicecat::server)
+    target_compile_features(test_dred_toggle PRIVATE cxx_std_20)
+    target_include_directories(test_dred_toggle PRIVATE ${VC_TEST_INTERNAL_INCLUDES})
+    add_test(NAME dred_toggle COMMAND test_dred_toggle)
+    set_tests_properties(dred_toggle PROPERTIES TIMEOUT 60)
 endif()