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>

firmware/include/audio_config.h

@@ -0,0 +1,14 @@
+// SPDX-License-Identifier: GPL-3.0-only
+// Recording parameters. Defaults: 16 kHz mono 16-bit PCM (see state/DECISIONS.md).
+#pragma once
+
+#define OSC_SAMPLE_RATE     16000
+#define OSC_BITS_PER_SAMPLE 16
+#define OSC_CHANNELS        1
+
+// Directory on the SD card for recordings + sidecar metadata.
+#define OSC_REC_DIR         "/recordings"
+
+// Capture chunk size (bytes) pumped from the mic to the WAV file each update().
+// 4096 bytes = 2048 samples = 128 ms at 16 kHz mono 16-bit. Sits in a stack buffer.
+#define OSC_CHUNK_BYTES     4096

firmware/include/pins.h

@@ -5,10 +5,17 @@
 
 #if defined(OPENSCRIBE_BOARD_XIAO)
 // Seeed XIAO ESP32-S3 Sense: onboard PDM mic + microSD use the Sense board's fixed pins.
-// The audio driver selects the PDM path for this board; these are placeholders for the
-// user-facing controls only.
+// The audio driver selects the PDM path for this board.
+#define PIN_PDM_CLK     42  // XIAO ESP32-S3 Sense onboard PDM mic clock
+#define PIN_PDM_DIN     41  // XIAO ESP32-S3 Sense onboard PDM mic data
+// XIAO Sense onboard microSD (SDMMC-on-SPI fixed pins)
+#define PIN_SD_CS       21
+#define PIN_SD_SCK      7
+#define PIN_SD_MOSI     9
+#define PIN_SD_MISO     8
 #define PIN_BUTTON      2   // external tactile button to GND
-#define PIN_LED         21  // XIAO onboard user LED
+#define PIN_LED         LED_BUILTIN // XIAO onboard user LED (active-low)
+#define PIN_LED_ACTIVE_LOW 1
 #define PIN_VBUS_SENSE  -1  // use USB-serial/VBUS API on XIAO
 #define PIN_CHARGE_STAT -1
 
@@ -26,7 +33,8 @@
 
 // Controls / status
 #define PIN_BUTTON      7   // active-low, internal pull-up
-#define PIN_LED         48  // WS2812 data (or plain LED via 330R)
+#define PIN_LED         48  // plain LED via 330R (WS2812 support added later)
+#define PIN_LED_ACTIVE_LOW 0
 #define PIN_CHARGE_STAT 14  // TP4056 STAT (open-drain)
 #define PIN_VBUS_SENSE  15  // USB 5V present -> on-charge / powered mode
 #define PIN_MOTOR       16  // optional haptic, via transistor