DFU flashing

Reflash a board over plain USB — no debug probe:

bazel run //boards/VCU:dfu

dfu.bzl fetches pinned dfu-util 0.11 binaries for the host OS as @dfu (with a Firebase mirror, since SourceForge likes to flake). flash.py is the bazel run entry point (wrapped per-board by dfu_flash_target in tools/firmware). The flow:

1. Scan serial ports for the running firmware (description containing lhre, VID:PID 0483:5740, or Windows' generic "USB Serial Device") and send it update.\n — firmware built with enable_dfu = True reboots into the ST system bootloader on that command.
2. Find the DFU device (0483:df11) and download the .bin to flash at 0x08000000, then :leave to boot the new image.

If no serial port matches (device already in DFU mode, e.g. via the BOOT0 pin), step 1 is skipped and it flashes directly.