Reach a board over Bluetooth: ble:// devices through bledev

[bdbarnett]

Every board command now takes -d ble://NAME, which reaches a board running pydevices' bledev.repl or bledev.filetransfer over Bluetooth with no serial port involved. It's modelled on the ws:// transport: a pyserial-shaped link handed to mpremote's SerialTransport, here over bleak in the sidecar's Python. The board side is PyDevices/pydevices#86 (unmerged).

export MPFTP_BLE_PASSWORD=...
mpftp exec -d ble://rack "print(1+1)"
mpftp put -d ble://rack main.py /main.py

• Files go over the board's BLE file-transfer service when it serves one (CircuitPython's protocol), and over the raw REPL when it doesn't. For 20 KB, file transfer took 0.83 s up and 0.30 s down; the raw REPL took 15.6 s and 10.6 s. put/get keep their SHA-256 check.
• The password is looked up the way the WebREPL one is (A WebREPL password set in VS Code can't be used by the CLI, the PWA or agents #43): first ~/.mpftp/webrepl-passwords.json under ble:NAME (mpftp wifi password ble://NAME), then MPFTP_BLE_PASSWORD / blePassword, then the WebREPL password, because bledev.repl defaults to webrepl_cfg.PASS.
• Resets. soft-reset refuses over BLE and says why: a soft reset turns Bluetooth off. soft-reboot works if main.py starts bledev. mount stays serial-only.

Gate, run on the T-Embed over BLE with USB serial used only for setup:

• CLI: ls, put (verified), get (cmp byte for byte), exec, and interrupt of a while True loop started by run (61 ms) all passed. The REPL answered afterwards.
• tools/ble_gate.py ran the same checks inside one sidecar session and passed.
• Planted faults: MPFTP_BLE_PLANT=put made put exit 1 with a hash mismatch, and MPFTP_BLE_PLANT=get made get report one.
• A wrong password, an absent board and soft-reset each gave a clear message.
• The unit tests pass (436 of 436, including 13 new ones in test_ble.py), and ruff is clean.

The measurements and design are in docs/plans/ble.md, and the numbers come from tools/ble_bench.py.

Not done

• The VS Code extension and the PWA don't list BLE boards.
• There's no ble find and no remembered BLE boards.
• Only Windows' radio has been tried.