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/NOTES.md

@@ -42,6 +42,16 @@ Per component (see each component's README for detail):
 - Do not put object-store credentials or WiFi passwords on the microSD in clear; they live
   in ESP32 NVS.
 
+## Firmware specifics (M1)
+
+- I2S/PDM via the core-bundled `ESP_I2S` (`I2SClass`) - no extra lib_deps. Dev board uses
+  `I2S_MODE_STD` (INMP441/ICS-43434); XIAO uses `I2S_MODE_PDM_RX` (onboard mic).
+- SD uses a dedicated HSPI bus (`SPIClass(HSPI)`) so it does not clash with other SPI use.
+- WAV writer streams a 44-byte header then patches RIFF/data sizes on close by seeking.
+- Recording ids are uptime-based until NTP lands (M2); metadata `started_at` is null until
+  then. See the follow-ups in TODO.md.
+- Not yet compiled locally (no PlatformIO on the dev host) - CI builds both profiles.
+
 ## Dead ends
 
 - (none yet)