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>

docs/SECURITY_PLAN.md

@@ -107,5 +107,6 @@ Everything else is checked with the `esp32devdbg` build, plus curl and websocket
 ## Not in this plan
 
 - TLS.
+- Checking GitHub's certificate in `GitOTA`. It calls `setInsecure()`, so anyone who can intercept the update download can serve their own firmware. ESP-IDF 5.5 ([ADR 0003](adr/0003-pioarduino-idf55.md)) ships a certificate bundle, so this is possible now. It needs its own change: the full bundle costs more flash than the ESP32 build has spare, and pinning GitHub's root certificates might fit.
 - Session tokens with expiry.
 - Contents of the binary backup file (it holds no passwords, but it does hold rolling codes; `/backup` becomes a config route).

docs/adr/0003-pioarduino-idf55.md

@@ -0,0 +1,18 @@
+# Firmware builds on pioarduino (Arduino 3.x, ESP-IDF 5.5), pinned to one release
+
+We moved every firmware env from `espressif32 @ 6.13.0` (Arduino 2.0.17 on ESP-IDF 4.4.7) to pioarduino's `55.03.312` (Arduino 3.3.12 on ESP-IDF 5.5). IDF 4.4 no longer gets security fixes for Wi-Fi, lwIP and mbedTLS, and the planned RMT radio needs IDF 5's RMT driver. The official PlatformIO platform never shipped Arduino 3.x, so pioarduino, a community fork, is the only way to get it in PlatformIO. We pin one release URL rather than `stable`, so a framework upgrade is always its own PR with its own size check and hardware test.
+
+## Considered Options
+
+- **Stay on IDF 4.4.** Rejected: no more security fixes, and no new RMT driver.
+- **Rebuild the IDF libraries with `custom_sdkconfig`** to leave out PPP, NAT and provisioning (about 20 KB the firmware never uses). Deferred: it makes builds much slower, and log level 2 already makes the ESP32 build fit.
+- **Grow the app slots in the partition table.** Rejected: an OTA update can't change the partition table, so every existing device would need a USB flash.
+
+## Consequences
+
+- **Firmware size.** The ESP32 image grows from 1,382,640 bytes (v3.0.14) to 1,555,472 at `CORE_DEBUG_LEVEL=2`, leaving 17 KB of the 1.5 MB OTA slot. At level 3 it doesn't fit, so release envs build at level 2 (warnings and errors) and debug envs stay at 3. PlatformIO's "Flash:" figure counts ELF sections, not the image, and reads about 45 KB low, so CI checks `firmware.bin` against the app partition and fails under 8 KB free (`.github/size_check.py`).
+- **No more C3 builds.** Its image grows by 214 KB to 1,651,216 bytes, 76.5 KB over the OTA slot even at level 2. C3 boards stay on the 3.0.x releases.
+- **Ethernet PHY ids are ours.** Arduino 3.x renumbered `eth_phy_type_t` (`ETH_PHY_GENERIC` is 0). NVS, backups and the web UI keep the Arduino 2.x numbers as `eth_phy_id_t` (`src/EthPhy.h`), translated by name when Ethernet starts. Never cast a stored id to the framework enum.
+- **Ethernet only on the original ESP32.** Arduino 3.x has the RMII MAC API only where `CONFIG_ETH_USE_ESP32_EMAC` is set. S3 and C6 build without Ethernet.
+- **Updates roll back until proven.** We override the core's `verifyRollbackLater()` hook to return true, and `loop()` marks the new firmware valid once it has run 60 s and the network is up. A reset before then boots the previous firmware. The LittleFS image an update wrote stays, so the old firmware runs with the new web UI.
+- **Tooling.** pioarduino needs its own Core fork (`pip install pioarduino`, the pioarduino IDE extension in VS Code), which it installs into `~/.platformio/penv` in place of PlatformIO Core.