build: add the ESP32-C5

The C5 builds like the C6: a debug build with one 3 MB app slot, so it
updates over USB. At CORE_DEBUG_LEVEL=2 its image left 3.4 KB of the
1.5 MB OTA slot, under CI's 8 KB.

- platformio.ini: env esp32c5 (esp32-c5-devkitc-1).
- Chip model "c5", so the updater and releases name its files
  SomfyController.*.esp32c5.bin.
- Radio defaults: the framework's SPI pins (CSN 6, MOSI 8, MISO 9,
  SCK 10), TX 4, RX 5.
- Web UI: GPIO 0-28, without 13-14 (USB) and 15-22 (flash, PSRAM).
- CI builds and releases it; its bootloader sits at 0x2000.
- ADR 0003: amended.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

platformio.ini

@@ -103,6 +103,18 @@ build_flags =
 	${esp32.build_flags}
 	-DCORE_DEBUG_LEVEL=2
 
+; Released as a debug build with one 3 MB app slot, so C5 and C6 boards
+; update over USB, not OTA.  The C5's image leaves 3.4 KB of the 1.5 MB OTA
+; slot even at level 2.
+[env:esp32c5]
+extends = esp32
+board = esp32-c5-devkitc-1
+build_type = debug
+board_build.partitions = esp32_huge_app.csv
+build_flags =
+	${esp32.build_flags}
+	-DCORE_DEBUG_LEVEL=3
+
 ; Released as a debug build with one 3 MB app slot, so C6 boards update over
 ; USB, not OTA.
 [env:esp32c6]