fbuild-serial: USB-level bootloader re-enumeration detection (complement to #688 serial-line classifier)

[zackees]

TL;DR

#688 proposes a BootModeClassifier registry for recognizing wedged-device signatures on the serial stream. That covers serial-visible failures: ESP ROM download, ARM HardFault, etc. It does not cover USB-level bootloader re-entry — when a board transitions to a bootloader by changing its USB VID:PID and presenting a new interface.

Concretely:

• RP2040 BOOTSEL → board disappears from the host as a CDC serial device and reappears as a USB MSC drive (2E8A:0003 Bootrom). Without USB-level awareness, fbuild sees "monitor died" and retries, never noticing the bootloader is open.
• SAMD21 / SAMD51 DFU → similar: native USB CDC drops, USB DFU interface appears (Atmel 03EB:6124).
• Teensy HID bootloader → similar: serial drops, HID 16C0:0478 interface appears.

This is a different problem from #688 (no serial line to classify) and a different problem from #687's BoardFamily (#687 says "this is how to enter bootloader," not "I observed bootloader entry").

Proposed shape

A small USB-device watcher on top of serialport::available_ports() (or platform-specific equivalents) that:

1. Snapshots the set of USB devices at deploy start.
2. After triggering bootloader entry (e.g. TouchBaud1200), watches for the expected new VID:PID with a timeout.
3. Returns a typed event (BootloaderEntered { vid, pid, device_path }) that the deployer consumes.

pub enum BootloaderSignature {
    Rp2040BootSel,                  // 2E8A:0003 USB MSC
    SamdDfu,                        // 03EB:6124 USB DFU
    SamdMassStorage { class: u8 },  // SAMD-CIRCUITPY etc.
    TeensyHidBootloader,            // 16C0:0478
    EspRomDownloader,               // already a serial-line concern; flag both
}

pub fn watch_for_bootloader(
    expected: BootloaderSignature,
    timeout: Duration,
) -> Result<DeviceHandle>;

This is the missing piece for "we touched 1200 baud and the port disappeared; let's confirm the bootloader is actually there before we try to flash."

Acceptance criteria

• bootloader_watcher module in fbuild-serial or fbuild-deploy.
• At least one signature implemented + unit-tested: RP2040 BOOTSEL.
• TouchBaud1200 reset path in #687 consumes the watcher and returns a typed error if the bootloader doesn't appear within the timeout — instead of silently retrying.
• CI test: stubbed USB device snapshot with the expected VID:PID inserted at T+1s; watcher returns success.

Refs

• FastLED/fbuild#687 — BoardFamily + ResetMethod. TouchBaud1200 consumer of this.
• FastLED/fbuild#688 — BootModeClassifier for serial-line wedges. Complementary, not duplicative.
• FastLED/fbuild#686 — BOARD_FINGERPRINTS. Bootloader VID:PIDs go in the same table.

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