refactor: complete hsmc conversion and add debug-checkpoint feature

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 = ()`.

src/host/uart.rs

@@ -67,11 +67,16 @@ pub async fn host_task(
                 }
             }
             Either::Second(response) => {
+                #[cfg(feature = "debug-checkpoint")]
+                crate::debug_blink::set(13);
                 if let Some(frame) =
                     framing::cobs_encode_response(response, &mut write_buf, &mut cobs_encode_buf)
                 {
                     if tx.write_all(frame).await.is_err() {
                         warn!("UART write failed, response dropped");
+                    } else {
+                        #[cfg(feature = "debug-checkpoint")]
+                        crate::debug_blink::set(14);
                     }
                 }
             }