Finishes the hsmc migration that was staged across the prior two
commits:
- Radio task is now a single `machine.run().await`; all I/O lives in
state-scoped `during:` activities (`next_command`, `next_packet`,
`perform_tx`). The old `push/drain` helpers, the imperative
top-level `select`, `handle_cmd`, `handle_rx_result`, and
`execute_transmit` are gone. `StartingRx` / `ResumingRx` intermediate
states model the host-visible difference between a responding and a
silent RX re-arm.
- Host/USB task is now an hsmc statechart (Disconnected/Connected)
with entry/exit actions handling display wake, radio StopRx, and
display reset — replacing the open-coded `was_connected` edge
detector. The `protocol_loop` keeps its imperative read path because
`read_packet` blocks.
- New `debug-checkpoint` cargo feature ships a dedicated
`debug_blink.rs` task and a shared atomic counter that code along
the boot/dispatch/TX path bumps at notable points. A ladder-latching
write policy preserves the deepest TX phase reached on the first
attempt until reboot — diagnostic output for UART-only boards where
defmt would corrupt the host protocol. Zero footprint when the
feature is off.
- Silence a stray `clippy::unit_arg` on `led_off` / `led_blip` in the
display task for boards with `LedDriver = ()`.
