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>

.github/workflows/build.yaml

@@ -60,9 +60,7 @@ jobs:
         # fwname: firmware-only binary for OTA updates
         # obname: onboard image (bootloader + partitions + firmware + littlefs merged)
         #         for flashing a new chip via USB/serial
-        # addr_bootloader: chip-dependent (ESP32: 0x1000, S3/C6: 0x0)
-        # No C3: on ESP-IDF 5.5 its image no longer fits the 1.5 MB OTA slot
-        # (docs/adr/0003-pioarduino-idf55.md).  C3 boards stay on 3.0.x.
+        # addr_bootloader: chip-dependent (ESP32: 0x1000, S3/C3/C6: 0x0)
         # addr_fs: must match spiffs/littlefs offset in esp32_3MB.csv
         include:
           - env: esp32dev
@@ -81,6 +79,14 @@ jobs:
             fwname: SomfyController.ino.esp32s3.bin
             fsname: SomfyController.littlefs.esp32s3.bin
             obname: SomfyController.onboard.esp32s3.bin
+          - env: esp32c3
+            name: ESP32-C3
+            chip: ESP32-C3
+            addr_bootloader: "0x0"
+            addr_fs: "0x310000"
+            fwname: SomfyController.ino.esp32c3.bin
+            fsname: SomfyController.littlefs.esp32c3.bin
+            obname: SomfyController.onboard.esp32c3.bin
           - env: esp32c6
             name: ESP32-C6
             chip: ESP32-C6