build: bring back the ESP32-C3 build

With the old API gone, the C3 image is 1,498,512 bytes at
CORE_DEBUG_LEVEL=2 and leaves 72.6 KB of its 1.5 MB OTA slot. It was
76.5 KB over when ESP-IDF 5.5 dropped it.

- platformio.ini: env esp32c3, single core, esp32_3MB.csv, level 2.
- CI builds and releases it with the other chips.
- ADR 0003: amended.

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

platformio.ini

@@ -95,6 +95,14 @@ build_flags =
 	${dualcore.build_flags}
 	-DCORE_DEBUG_LEVEL=2
 
+[env:esp32c3]
+extends = esp32
+board = esp32-c3-devkitm-1
+board_build.partitions = esp32_3MB.csv
+build_flags =
+	${esp32.build_flags}
+	-DCORE_DEBUG_LEVEL=2
+
 ; Released as a debug build with one 3 MB app slot, so C6 boards update over
 ; USB, not OTA.
 [env:esp32c6]