feat(firmware): implement hw_ble_stop() — disable BLE during deep sleep

[jknoxdev]

hw_ble_stop() in firmware/src/main.c:343 is currently a stub.
It is called by fsm_hw_enter_deep_sleep() before the RTC sleep
interval, but BLE stays active — the node never actually stops
advertising during deep sleep cycles.

Implementation:
Replace the stub body with a real bt_disable() call. Pair with
a matching bt_enable() on RTC wakeup before the FSM transitions
back to ARMED.

Acceptance:
PPK2 or scope shows BLE current draw drops during deep sleep window.

label: enhancement