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>

PROGRESS.md

@@ -39,7 +39,8 @@ up instantly. Newest status at the top.
     `allowBluetoothA2DP` and the output route must remain the headphones/A2DP device — NOT flip
     to `…Record`. If confirmed, delete the `logSessionState` calls + method and the prior
     band-aid comments in `IOSAudioRouter`/`audio_engine.cpp` can be trimmed.
-  - **Verified on Windows:** `cmake --build --preset dev` clean, `ctest --preset dev` 21/21.
+  - **Verified on Windows:** `cmake --build --preset dev` clean, `ctest --preset dev` 22/22
+    (21/21 prior + new `test_dred_toggle`).
     iOS build & on-device run still to be done by the user on the Mac.
 
 - **Planned (not started):** **External PCM feed/tap API (`vc_stream_feed_pcm` +
@@ -393,7 +394,19 @@ iOS 18.0 deployment target. App Group `group.cat.voice.VoiceCat` for Keychain sh
 - [x] **All three client UIs** (Windows WinForms, macOS AppKit, iOS SwiftUI) expose the full
   M5 moderation and admin surface.
 - [x] **Docs** — `docs/protocol.md`, `docs/security.md` kept in sync.
-- [ ] **DRED/audio-quality polish** — not started.
+- [x] **DRED/audio-quality polish** — done (2026-06-20). `bool dred` added to `AudioConfig`
+  proto (field 11) and `vc_audio_config` C ABI. Encoder: `OPUS_SET_DRED_DURATION(2)` when
+  enabled (20 ms of ML redundancy per packet). Decoder: `OpusDREDDecoder` + per-stream
+  `OpusDRED` scratch pre-allocated; `JitterBuffer::try_copy_front_payload` peeks at the next
+  buffered packet on every PLC step; if DRED data is present, `opus_decoder_dred_decode`
+  reconstructs the lost frame — otherwise falls back to standard PLC. New test:
+  `test_dred_toggle` (ctest 22/22). Files: `voicecat.proto`, `voicecat.h`,
+  `opus_codec.{h,cpp}`, `audio_engine.{h,cpp}`, `client.cpp`, `session.{h,cpp}`.
+- [ ] **DRED toggle in client UIs** — expose the `dred` flag in all three channel-config UIs
+  so admins can enable it per channel. Windows: `ChannelEditForm` / `vc_channel_info.audio.dred`
+  checkbox. macOS AppKit: channel-edit sheet. iOS SwiftUI: channel-edit form. All three UIs
+  already have full channel CRUD wired; this is an additive checkbox on the existing audio-config
+  section. (Core/protocol/ABI all done — this is UI-only work.)
 - [ ] **macOS ScreenCaptureKit screen-audio** — `startStream(.screenAudio)` in macOS client
   announces the stream but `start_loopback_capture()` returns false (no `VOICECAT_HAS_LOOPBACK`
   on macOS). Implement via `vc_stream_feed_pcm` + `SCStream` once the feed API ships.