feat: SD card firmware update (OTA from removable media)

[dougborg]

Summary

Support firmware self-update from an SD card at boot. Place airhound.bin on the SD card, insert it, and the device flashes itself. Simple but effective for field-deployed sensors without network connectivity.

Inspired by ESP32DualBandWardriver's SD card OTA update mechanism.

Design

1. On boot, check SD card root for airhound.bin (or versioned: airhound-v1.2.3.bin)
2. Verify firmware image integrity (SHA-256 checksum, or signature for supply-chain safety)
3. Flash to the inactive OTA partition (ESP-IDF A/B partition scheme for rollback safety)
4. Mark new partition as boot target
5. Delete or rename the .bin file to prevent reflash loop
6. Reboot into new firmware

Progress Indication

• M5StickC: Display flash progress (percentage bar)
• XIAO: LED blink pattern during flash
• Both: Serial log output for debugging

Hardware Requirements

• Requires SD card slot — not all boards have one natively
  • XIAO ESP32-S3 has microSD via expansion board
  • M5StickC Plus2 does not have SD (would need external SPI SD module)
• SPI SD card driver needed (or SDMMC on supported chips)

Safety

• A/B OTA partitions ensure rollback if new firmware fails to boot
• Checksum verification prevents flashing corrupt images
• Optional: require a signature file (airhound.bin.sig) for verified updates
• Boot count watchdog: if new firmware crashes N times, revert to previous

Relates To

• arch: evaluate ESP-IDF (std) migration for ESP32 firmware #18 — ESP-IDF evaluation (OTA APIs are ESP-IDF)
• Add 4 operating modes with NVS persistence #84 — firmware update infrastructure
• Add WiFi AP + STA mode support #86 — WiFi AP mode (complementary OTA path — network vs. SD card)
• Add NeoPixel LED driver #87 — update mechanism planning