bledev: the P4 HID-host gate, and a board typing in a passkey

[bdbarnett]

Stacked on #88. Two things from the hardware proofs #83 and #88 left owed.

The P4 as HID host passes on the flow-control image. p4_hid_gate.py repeats #83's gate with the LCD-7 typing into the P4, which runs micropython-pydevices#12. At the default interval, 10 of 10 runs passed with 112 of 112 events. At 7.5-15 ms, 10 of 10 completed runs passed (one more attempt never connected). The planted lost Shift failed at event 0. Before #12 the P4 lost keystrokes in 3 of 5 and 6 of 6 runs. The numbers are on #87.

A board can't type in a passkey on stock MicroPython. The LCD-7 showed its passkey and the P4 (as central) was never asked for it: no _IRQ_PASSKEY_ACTION arrived, and pairing failed at NimBLE's 30 s timeout. MicroPython's peripheral_gap_event_cb (the callback for connections a board makes) drops BLE_GAP_EVENT_PASSKEY_ACTION. That's true on every NimBLE board, not only the P4, and upstream's draft micropython/micropython#19470 moves the event. The fake radio couldn't see this.

The first commit works around it in bledev.mpble. pair(passkey=...) waits 1.5 s for the stack to ask. If it hasn't, bledev asks the provider itself and keeps offering the passkey with gap_passkey() until NimBLE takes it (NimBLE answers EINVAL until the pairing reaches its confirm step). Proven on hardware (2026-09-24), on the P4's stock #12 image with no firmware patch. Right passkey: 3 of 3 runs paired, authenticated, bonded and moved 20 KB byte for byte. Wrong passkey: 3 of 3 refused, no bond on either board. The planted justworks run failed. The fake-radio suites pass (test_bledev, _pairing, _filetransfer, _hid).

The second commit adds the rigs:

• uart_rig.py drives the boards' friendly REPLs over their UARTs, which keeps ESP-IDF's log visible and allows a hard reset between P4 runs (micropython-pydevices#13).
• p4_hid_gate.py runs the HID gate with the P4 as host, and --mode connect (with connect_loop_server.py/connect_loop_client.py) counts connect timeouts.
• passkey_gate.py carries a passkey the way a person would: it reads the digits off the serving board's console and types them into the other board's, where files_pair_client.run(passkey="console") reads them with input().

[bdbarnett]

The workaround is now proven on hardware, with the P4 on its stock #12 image and no firmware patch. Right passkey: 3 of 3 runs paired, bonded and moved 20 KB byte for byte, then reconnected from the bond with no second passkey. Wrong passkey: 3 of 3 refused, with no bond left on either board. The planted justworks run failed, as it must. The details are on #88: #88 (comment)