feat(firmware): M1 recording core - mic capture to WAV on SD

Implements the first working feature: the device records audio to the microSD
card, toggled by the button, with LED status and per-recording JSON metadata.

What changed:
- firmware/src/audio.{h,cpp}: mic capture via the core-bundled ESP_I2S. Dev board
  uses I2S standard mode (INMP441/ICS-43434); XIAO uses PDM mode (onboard mic).
  Presents 16-bit PCM mono to callers regardless of board.
- firmware/src/storage.{h,cpp}: microSD on a dedicated HSPI bus + a streaming
  WavWriter that writes a 44-byte PCM header and patches RIFF/data sizes on close;
  plus sidecar JSON metadata writer.
- firmware/src/recorder.{h,cpp}: idle/recording state machine; creates
  /recordings/<id>.wav, pumps mic chunks in on update(), finalises + writes
  <id>.json (Recording schema from api/openapi.yaml) on stop.
- firmware/src/ux.{h,cpp}: debounced button (short press toggles) + status LED
  patterns (idle/recording/error), active-low aware.
- firmware/src/main.cpp: wires ux + recorder; loop toggles on button and drains
  the mic while recording.
- firmware/include/audio_config.h: 16 kHz mono 16-bit, chunk size, rec dir.
- firmware/include/pins.h: added XIAO PDM mic + onboard SD pins, LED active-low flag.
- state/: TODO, ARCHITECTURE, NOTES updated for M1 and the deferred follow-ups.

Why:
- Recording is the foundation every later milestone (transfer, upload, transcription)
  builds on. Kept dependency-free (only core-bundled ESP_I2S + SD) for simple CI builds.

Notes:
- Not compiled locally (no PlatformIO on the dev host) or hardware-verified; Forgejo
  Actions CI builds both board profiles. Follow-ups tracked in state/TODO.md:
  INMP441 16-bit level calibration, and real NTP timestamps (ids are uptime-based
  until M2 brings WiFi/NTP).

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

state/TODO.md

@@ -5,13 +5,13 @@
 
 ## In progress
 
-- [ ] M0 Scaffold - repo, licences, state docs, BOM, OpenAPI, component skeletons, CI
-      (branch: bootstrap on `main`; this is the setup commit)
+- [ ] M1 Firmware recording core - code complete on `feature/fw-recorder-core`, in PR.
+      I2S/PDM mic capture -> WAV on microSD, button start/stop, LED status, sidecar JSON
+      metadata. Not yet compiled locally (no PlatformIO here) or hardware-verified; relies
+      on Forgejo Actions CI to build both board profiles.
 
 ## Pending (roughly in build order)
 
-- [ ] M1 Firmware recording core: I2S mic capture -> PSRAM ring buffer -> WAV on microSD,
-      button start/stop, LED status, sidecar JSON metadata. (branch `feature/fw-recorder-core`)
 - [ ] M2 On-device WiFi + REST API: WiFi manager, mDNS, list/get/download(range)/delete
       recordings, device status. Implements `api/openapi.yaml` device paths.
       (branch `feature/fw-rest-api`)
@@ -34,12 +34,22 @@
 
 ## Done
 
+- [x] M0 Scaffold - repo, licences, state docs, BOM, OpenAPI, skeletons, CI
+      (merged to `main`, 2026-07-03)
 - [x] Create Forgejo repo `laurence/openscribe` (public, main) - 2026-07-03
 - [x] Decide hardware, sync model, AI stack, app platform, storage, licensing (see
       DECISIONS.md) - 2026-07-03
 
+## Follow-ups (deferred, do when convenient)
+
+- [ ] Audio calibration: INMP441 emits 24-bit samples in a 32-bit slot. M1 reads in
+      16-bit mode (usable for speech, may be quiet). If levels are low on the dev build,
+      capture 32-bit and right-shift into int16 in `audio.cpp`.
+- [ ] Real timestamps: M1 ids/`started_at` are uptime-based (no RTC/NTP yet). M2 adds NTP
+      over WiFi; switch ids and `started_at` to real UTC then.
+
 ## Blocked / waiting
 
 - [ ] Physical build + on-device verification - waiting on parts from `hardware/BOM.md`.
-      Until parts arrive, firmware is developed against the ESP32-S3 target and validated
-      by build + (where possible) native/host unit tests, not on real hardware.
+      Until parts arrive, firmware is validated by CI build (Forgejo Actions) of both
+      board profiles, not on real hardware.