build: add the ESP32-P4 (#29)

For boards such as Waveshare's ESP32-P4-WIFI6-POE-ETH: WiFi through the
board's ESP32-C6 on SDIO (esp-hosted), Ethernet on the P4's RMII MAC with
an IP101.  Not tested on a device: no P4 board was at hand.

- platformio.ini: env esp32p4 (esp32-p4_r3, chip v3.01+) and esp32p4es
  (esp32-p4, the esp32p4_es libraries, chips before v2.0).  An image runs
  on one of the two only, so each has its own release files, chip models
  "p4" and "p4es".  Both keep the OTA slot: at level 2 they leave about
  128 KB of it.
- Ethernet clock mode: stored as eth_clock_id_t in the ESP32's numbering
  and translated by name, like the PHY id.  The P4's eth_clock_mode_t is
  the IDF's emac_rmii_clock_mode_t, whose 0 is a deprecated default; only
  clock in (GPIO 50) translates there.
- P4 Ethernet defaults: the Waveshare wiring (IP101 as TLK110, address 1,
  MDC 31, MDIO 52, reset 51), board type 8.  The ESP32's put MDIO on an
  SDIO pin.
- Settings take pins up to GPIO 54, the P4's last, instead of 48.
- Radio defaults: SPI 32/33/27, CSN 46 (the framework's Waveshare
  variant), TX 2, RX 3.
- Web UI: a Waveshare ESP32-P4-ETH preset, the P4's clock mode, pin lists
  up to GPIO 54 without the pins the C6 link, Ethernet, USB and UART0 use.
- CI builds and releases both, with their ELFs; bootloader at 0x2000.
- ADR 0003: amended.

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

docs/adr/0003-pioarduino-idf55.md

@@ -15,7 +15,8 @@ We moved every firmware env from `espressif32 @ 6.13.0` (Arduino 2.0.17 on ESP-I
 
   > **Amended by [ADR 0006](0006-two-security-modes-eleven-http-routes.md) (4.0.0).** The C3 builds again. Without the old API (ports 8080 and 8081), its image is 1,498,512 bytes at level 2 and leaves 72.6 KB of the OTA slot. It runs on chip revision v0.3 or later only: the framework's C3 libraries are built with `CONFIG_ESP32C3_REV_MIN_3`. Its default radio pins moved off GPIO 12-17, which hold the SPI flash.
 - **The C5 is updated over USB.** New in 4.0.0. At level 2 its image left 3.4 KB of the OTA slot, under CI's 8 KB, so like the C6 it is released as a debug build with one 3 MB app slot (`esp32_huge_app.csv`), which no OTA can write. The framework's C5 libraries run on chip revision v1.0 or later only (`CONFIG_ESP32C5_REV_MIN_FULL=100`). Its WiFi has 5 GHz too, so the passive scans for the configured network (`src/ESPNetwork.cpp`, 300 ms a channel) cover about 38 channels instead of 13: about 11 s instead of 4 s, depending on the country's channels.
-- **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, C3, C5 and C6 build without Ethernet.
+- **The P4 has two builds.** New in 4.0.0, for boards such as Waveshare's ESP32-P4-WIFI6-POE-ETH. The framework's P4 libraries run on chip revision v3.01 or later only (`CONFIG_ESP32P4_REV_MIN_FULL=301`), its `esp32p4_es` libraries on revisions before v2.0, so `esp32p4` (chip model `p4`) and `esp32p4es` (`p4es`) each get their own release files. Both keep the OTA slot: at level 2 their images leave about 128 KB. WiFi goes through the board's ESP32-C6 on SDIO (esp-hosted, the framework's pins 14-19 and reset 54), so the C6 has to run a matching esp-hosted slave firmware, which Waveshare's boards ship with.
+- **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. The RMII clock mode is kept the same way, as `eth_clock_id_t` in the ESP32's numbering: the P4's `eth_clock_mode_t` is the IDF's `emac_rmii_clock_mode_t`, whose 0 is a deprecated default. On the P4 only clock in (GPIO 50) translates, since `ETH.begin()` keeps the clock on that pin.
+- **Ethernet only on the original ESP32 and the P4.** Arduino 3.x has the RMII MAC API only where `CONFIG_ETH_USE_ESP32_EMAC` is set. S3, C3, C5 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.