fbuild-deploy: enumerate supported deploy protocols per board; fail fast on unsupported #692

TL;DR

Today's fbuild deploy stack accepts a deploy method (esptool, pyocd, 1200bps, future dfu-util, etc.) and tries it; if the board doesn't support that method, the failure surfaces as a 30-180s timeout or a backend-specific error that doesn't say "this board doesn't do OTA." For a user who picks the wrong upload_protocol, the experience is "it hangs."

This issue asks for explicit per-board enumeration of supported deploy methods and a fail-fast guard up front.

Where this came from

The audit on FastLED/FastLED#3339 noted that:

• ESP32 supports OTA, esptool, esp-jtag.
• LPC845-BRK supports CMSIS-DAP/pyocd only — no OTA, no esptool.
• Teensy supports HID bootloader only — no OTA.
• STM32 with USART boot ROM: DFU + USART, no native OTA.
• RP2040: BOOTSEL → MSC drag-drop, OR picotool DFU, no OTA.

A bash autoresearch lpc845brk --use-ota attempt today hangs trying esptool or just silently doesn't work depending on which path is taken. The user expectation is "fail fast with a clear message."

Proposed shape

PlatformIO board JSON already has upload.protocol + upload.protocols (array). fbuild reads it. Add a pre-deploy filter:

fn validate_deploy_method(board: &BoardConfig, requested: DeployMethod) -> Result<()> {
    let supported = board.upload.protocols.clone();
    if !supported.contains(&requested.as_str().to_string()) {
        return Err(FbuildError::UnsupportedDeployMethod {
            board: board.name.clone(),
            requested,
            supported,
        });
    }
    Ok(())
}

Error message:

Error: deploy method 'ota' is not supported on board 'lpc845brk'.
       Supported: cmsis-dap, mbed.
       Use `bash autoresearch lpc845brk` (default: cmsis-dap) instead of
       `--use-ota`.

Acceptance criteria

• validate_deploy_method runs in the top-level orchestrator before any backend is selected.
• Error carries board, requested, supported so the user gets all three in one read.
• CI test that picks an unsupported method per board family and asserts the right error.
• Default --use-XXXX flag → board's first supported protocol when no flag is passed.

Refs

• FastLED/fbuild#686 — BOARD_FINGERPRINTS. Same data source (board JSON).
• FastLED/fbuild#687 — BoardFamily. The reset_method() API already gates on family; this issue applies the same pattern to deploy method.

Filed from the audit fan-out on FastLED/FastLED#3339.