docs(adr-0005): Phase 3.5 dual-channel capture supersedes the masked-mono timeline (FR-SPK)

docs/adr/0005-diarization-sherpa-onnx.md

@@ -56,6 +56,30 @@ The original blind-clustering + voiceprint pass **remains the fallback** for mic
 imports (no timeline). Naming stays uniform (`build_name_map`: "You", then `Speaker 2…`). Segment
 IDs and the names-in-DB rule are unchanged; only the *source* of the spans changes.
 
+### Phase 3.5 refinement — dual-channel capture (supersedes Phase 3's timeline, 2026-07-14)
+
+The Phase 3 timeline above reconstructed the mic/far-side split *after the fact* from a summed-mono
+`audio.wav` plus a `mic_activity.json` sidecar, masking the mic ranges before clustering. That was
+reliable at stop but **fragile on reprocess** (re-aligning a sidecar against a mono mix — it
+collapsed to a single speaker) and carried a parallel persistence path.
+
+**Decision:** when the mic is enabled, record `audio.wav` as **stereo with the streams separated —
+left = microphone ("You"), right = system/loopback ("Speaker")** instead of summing them. The
+separation is then intrinsic to the recording:
+
+- Diarization runs on the **right channel only** → `Speaker N`; "You" comes from **left-channel**
+  voice activity (`vad_spans`). The mic is never clustered, by construction.
+- Reprocess recomputes both from the file — no sidecar, no masking; stop and reprocess agree.
+- Transcription still downmixes (L+R) to the same summed mono; playback folds to mono and bundle
+  export folds to dual-mono so shared/played audio is normal.
+- A `meetings.audio_layout` flag (`summed` | `split`, carried in the bundle manifest) distinguishes
+  recordings; existing `summed` recordings keep the blind-clustering + voiceprint path. On a stereo
+  render endpoint this is **size-neutral** (the file was already 2ch with the mic summed into both).
+
+This **retires** `MicActivity` / `mic_activity.json` / masked diarization from Phase 3 (see the
+retirement note at `attribute_split` in `commands.rs`); the capture-silence handling,
+`diarize_samples`, and `build_name_map` naming are retained. Segment IDs and names-in-DB unchanged.
+
 ## Revisit if
 A single model gives joint ASR + diarization with better accuracy, or whisper.cpp gains
 production diarization.