docs(data-model): add mic_activity.json to the meeting dir list (FR-SPK)

docs/03-data-model.md

@@ -16,6 +16,7 @@ Default root: `%LOCALAPPDATA%\WhispAssist\` (user-configurable, FR-STORE-2).
     └── <meeting_id>\           # one folder per meeting (uuid)
         ├── audio.wav           # canonical recording — present ONLY if "Record" was on (ADR-0009)
         ├── voiceprint.wav      # mic-only 16kHz sample for "You" re-matching on reprocess (FR-SPK); lives/dies with audio.wav
+        ├── mic_activity.json   # Phase 3 "You" spans (mic-speech timeline) for per-stream attribution on reprocess (FR-SPK); local-only, lives/dies with audio.wav
         ├── transcript.json     # canonical transcript (segments+speakers+timings)
         ├── manual_notes.json   # raw user-authored notes captured live during recording
         ├── notes.md            # the final notes document: manual notes + transcript, merged at finalize