docs(state): mark testing harness merged; M3 next

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

state/TODO.md

@@ -5,7 +5,7 @@
 
 ## In progress
 
-- [ ] Dev testing + emulation harness - on `feature/dev-testing`, in PR.
+- [ ] (nothing active - M3 is next)
 
 ## Pending (roughly in build order)
 
@@ -29,6 +29,8 @@
 
 ## Done
 
+- [x] Dev testing + emulation harness - native unit tests (wav/range), Wokwi emulator,
+      cppcheck + OpenAPI validation in CI, docs/testing.md (PR #3, 2026-07-03).
 - [x] M2 On-device WiFi + REST API - WiFi/SoftAP + mDNS + config(NVS) + device REST API
       (status, list, metadata, audio-with-Range, delete, record start/stop, config).
       (PR #2 `feature/fw-rest-api`, 2026-07-03). Not yet hardware-verified.