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/size_check.py

@@ -0,0 +1,52 @@
+"""Fail the build when the firmware leaves too little room in its app slot.
+
+PlatformIO only fails once the firmware no longer fits.  Releases are built at
+CORE_DEBUG_LEVEL=2 because the ESP32 image barely fits its 1.5 MB OTA slot, so
+this warns while there is still room to act.
+
+Usage: python .github/size_check.py <build dir> <name> [min free KB]
+Writes a line to $GITHUB_STEP_SUMMARY when it is set.
+"""
+import os
+import struct
+import sys
+
+ENTRY = struct.Struct("<HBBII16sI")  # magic, type, subtype, offset, size, label, flags
+ENTRY_MAGIC = 0x50AA
+APP_TYPE = 0x00
+
+
+def app_slot_size(partitions_bin):
+    """The largest app partition in a built partition table."""
+    with open(partitions_bin, "rb") as f:
+        table = f.read()
+    largest = 0
+    for pos in range(0, len(table) - ENTRY.size + 1, ENTRY.size):
+        magic, ptype, _, _, size, _, _ = ENTRY.unpack_from(table, pos)
+        if magic != ENTRY_MAGIC:
+            break
+        if ptype == APP_TYPE:
+            largest = max(largest, size)
+    return largest
+
+
+def main():
+    build_dir, name = sys.argv[1], sys.argv[2]
+    min_free = int(sys.argv[3]) * 1024 if len(sys.argv) > 3 else 8 * 1024
+    used = os.path.getsize(os.path.join(build_dir, "firmware.bin"))
+    slot = app_slot_size(os.path.join(build_dir, "partitions.bin"))
+    free = slot - used
+    ok = free >= min_free
+    line = (f"| {name} | {used:,} | {slot:,} | {free / 1024:.1f} KB | "
+            f"{'OK' if ok else f'FAIL: under {min_free // 1024} KB free'} |")
+    print(line)
+    summary = os.environ.get("GITHUB_STEP_SUMMARY")
+    if summary:
+        with open(summary, "a", encoding="utf-8") as f:
+            f.write("| Env | Firmware | App slot | Free | |\n|---|---|---|---|---|\n")
+            f.write(line + "\n")
+    return 0 if ok else 1
+
+
+if __name__ == "__main__":
+    sys.exit(main())