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>

README.md

@@ -7,6 +7,9 @@
   * Backup does no longer use LITTLEFS. Only RAM is used. Storage should not wear out so quick.
   * Added uptime info in web UI
 
+## Building
+The firmware builds on [pioarduino](https://github.com/pioarduino/platform-espressif32) (Arduino 3.x on ESP-IDF 5.5), not the stock PlatformIO platform. In VS Code use the **pioarduino IDE** extension instead of PlatformIO IDE. On the command line, `pip install pioarduino` gives you `pio`. Run the unit tests with `pio test -e native`. [ADR 0003](docs/adr/0003-pioarduino-idf55.md) explains why.
+
 ## There is a partition layout modification it's just better to perform full ESP32 wipeout. Perform backup then wipeout the firmware and restore from the bkp.
   * fwname: firmware-only binary for OTA updates
   * obname: onboard image (bootloader + partitions + firmware + littlefs merged) for flashing a new chip via USB/serial