build: move to pioarduino 55.03.312 (Arduino 3.3.12, ESP-IDF 5.5)

IDF 4.4 gets no more security fixes, and the planned RMT radio needs
IDF 5's RMT driver.  See docs/adr/0003-pioarduino-idf55.md.

- Every env on one pinned pioarduino release.  C6 drops its own
  library list and flags but stays a debug build on huge_app.
- Release envs build at CORE_DEBUG_LEVEL=2 so the ESP32 image fits its
  1.5 MB OTA slot (17 KB free); debug envs stay at 3.
- Drop the C3: its image is 76.5 KB over the OTA slot on IDF 5.5.
- CI installs pioarduino's Core and checks firmware.bin against the app
  partition, failing under 8 KB free.  PlatformIO's "Flash:" figure
  reads ~45 KB low and passed a C3 image that doesn't fit.
- esp_task_wdt_reconfigure(): on 3.x the core already starts the watchdog
  and esp_task_wdt_init() fails, so the C6 build kept the 5 s default.
- An updated firmware is kept only once it has run 60 s with the
  network up; a reset before then rolls back to the previous one.
- ETH.begin() takes the 3.x argument order; Arduino 2.x branches gone.

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

.github/workflows/build.yaml

@@ -21,8 +21,9 @@ jobs:
         with:
           python-version: "3.12"
 
+      # pioarduino's fork of PlatformIO Core, which the pioarduino platform needs.
       - name: Install PlatformIO
-        run: pip install platformio
+        run: pip install pioarduino
 
       - name: Run native unit tests
         run: pio test -e native
@@ -40,7 +41,9 @@ 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, C3/S3/C6: 0x0)
+        # 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_fs: must match spiffs/littlefs offset in esp32_3MB.csv
         include:
           - env: esp32dev
@@ -51,14 +54,6 @@ jobs:
             fwname: SomfyController.ino.esp32.bin
             fsname: SomfyController.littlefs.esp32.bin
             obname: SomfyController.onboard.esp32.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: esp32s3
             name: ESP32-S3
             chip: ESP32-S3
@@ -112,7 +107,7 @@ jobs:
           python-version: "3.12"
 
       - name: Install PlatformIO and esptool
-        run: pip install platformio esptool
+        run: pip install pioarduino esptool
 
       - name: Build LittleFS image
         run: pio run -e ${{ matrix.env }} -t buildfs
@@ -123,6 +118,9 @@ jobs:
       - name: Build firmware
         run: pio run -e ${{ matrix.env }}
 
+      - name: Check firmware size
+        run: python .github/size_check.py .pio/build/${{ matrix.env }} ${{ matrix.env }} 8
+
       - name: Restore LittleFS image
         run: cp littlefs.bin .pio/build/${{ matrix.env }}/littlefs.bin