refactor(radio): chart-first restructure on hsmc 0.3 + soak hardening

Migrate from hsmc 0.2 (local-patched) to hsmc 0.3.0 (crates.io, with
embassy + trace-defmt features). Restructure radio chart and bundle
several adjacent stability changes that need to land together for
soak runs.

Radio chart:
- Booting (default, no handlers) -> Operational (default Unconfigured;
  super-state owning CmdPing/CmdGetInfo/Reset/WedgeDetected handlers
  + Recovering + CheckRecoveryBudget) -> Dead (terminal).
- Cold init's during is structurally un-cancellable.
- radio_task is a 30-line shim that panics if run() returns (chart is
  non-terminating).
- Internal queue capacity bumped 8 -> 32 to cover action-emit bursts.

Host chart:
- Connected splits into WaitingForFirstFrame (default, no timer) +
  Active (chart-owned 1s inactivity timer via on(after) + self-
  transition on FrameReceived). Manual Timer::after + last_frame_at
  removed.

Soak hardening:
- ld/nrf52840-memory.x: app start 0x1000 -> 0x26000 to leave the
  factory s140 SoftDevice dormant in flash.
- hal/nrf52840.rs: dump POWER.RESETREAS on boot -- WARN on DOG/LOCKUP,
  INFO on RESETPIN/SREQ/wake-from-OFF.
- host/usb.rs: read_packet errors discriminated -- Disabled is debug
  (normal during USB enum), BufferOverflow stays warn.
- COBS encode warns now carry type/tag/payload-length for diagnosis.
- RADIO_THROTTLED static initializes to true (no proof of host drain
  at boot).
- driver/sh1106.rs: init retries 3x with 100ms backoff (intermittent
  first-write race on power-up).
- Cargo.toml: unexpected_cfgs allowlist for hsmc's journal/trace-*
  features.

docs/semtech_lora_cad_lbt_guidebook.md: vendor the Semtech CAD/LBT
reference for offline lookup.

src/host/framing.rs

@@ -54,7 +54,16 @@ pub async fn dispatch_frame(
 }
 
 /// Emit an async framing error (tag = 0x0000) per spec §2.5.
+///
+/// Backpressure: if the host transport's IN endpoint is observed-dead
+/// (RADIO_THROTTLED set by host_task), drop silently. Without this gate,
+/// a bad-frame burst from a wedged host fills `OutboundChannel` faster
+/// than host_task can drain it (because it can't drain it at all when
+/// write_packet keeps failing), producing the warn flood seen in soak.
 pub async fn emit_async_err(outbound: &OutboundChannel, code: ErrorCode) {
+    if crate::channel::RADIO_THROTTLED.load(portable_atomic::Ordering::Acquire) {
+        return;
+    }
     let frame = OutboundFrame {
         tag: 0,
         msg: DeviceMessage::Err(code),
@@ -106,15 +115,31 @@ pub fn encode_outbound(frame: &OutboundFrame, out: &mut [u8; MAX_WIRE_FRAME]) ->
     ) {
         Ok(n) => Some(n),
         Err(FrameEncodeError::BufferTooSmall) => {
-            warn!("encode_frame: output buffer too small");
+            warn!(
+                "encode_frame: output buffer too small (type={=u8}, tag={=u16}, payload={=usize}B, out_buf={=usize}B)",
+                frame.msg.type_id(),
+                frame.tag,
+                payload_len,
+                out.len(),
+            );
             None
         }
         Err(FrameEncodeError::PayloadTooLarge) => {
-            warn!("encode_frame: payload exceeds MAX_PAYLOAD_FIELD");
+            warn!(
+                "encode_frame: payload exceeds MAX_PAYLOAD_FIELD (type={=u8}, tag={=u16}, payload={=usize}B)",
+                frame.msg.type_id(),
+                frame.tag,
+                payload_len,
+            );
             None
         }
         Err(FrameEncodeError::CobsEncode) => {
-            warn!("encode_frame: COBS encode refused");
+            warn!(
+                "encode_frame: COBS encode refused (type={=u8}, tag={=u16}, payload={=usize}B)",
+                frame.msg.type_id(),
+                frame.tag,
+                payload_len,
+            );
             None
         }
     }