Follow-up audit after the ESP32 fix: classifyPin() was run for every board
against the protocol pin labels its element actually exposes. One real gap
remained -- Arduino Mega. Its dedicated SDA/SCL pins are only labelled (not
numbered), so I2C links drawn on them came back 'digital' and never bridged.
Map every Mega function label (TX/RX, TX0-3/RX0-3, SDA/SCL) to its pin number.
Audit result for the rest (added as board-protocols-audit.test.ts):
- Arduino Uno/Nano, Pico/Pico-W, STM32 Blue Pill: already OK.
- ESP32 / ESP32-C3: fixed earlier (esp32-uart-pin-classify).
- Raspberry Pi 3/4/5: OK -- the element labels pins by physical number (1..40)
which normalize to BCM, so no function-label gap exists there.
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