feat(mesh): operator-configurable Meshcore LoRa PHY params (freq/bw/sf/cr)

Archy set no radio params on Meshcore devices — SF/CR/BW/freq lived only in
device firmware, so a radio on the wrong preset could not be fixed from the
node (it hears RF energy but demodulates nothing; the fleet hit exactly this
on an RNode: docs/RETICULUM-TRANSPORT-PROGRESS.md item 4).

- protocol.rs: build_set_radio_params — wire format verified against the
  MeshCore companion firmware handler (CMD_SET_RADIO_PARAMS=11:
  [11][freq:u32 LE MHz*1000][bw:u32 LE kHz*1000][sf][cr]); byte-layout test
  pinned to the Portugal preset 869.618 MHz / 62.5 kHz / SF8 / CR8.
- serial.rs: MeshcoreDevice::set_radio_params (device reboots on OK).
- MeshConfig.lora_radio_params: Option — None (default) leaves the radio
  untouched, so only explicitly-configured deployments are affected.
- session.rs: provision on connect, gated on a persisted last-applied marker
  (SELF_INFO offsets shift across firmware versions) + the same attempt cap
  as region/channel so a refusing radio never reboot-loops.
- mesh.configure RPC: lora_radio_params arm with firmware-range validation.

mesh tests: 114/114 pass.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

core/archipelago/src/mesh/protocol.rs

@@ -210,6 +210,24 @@ pub fn build_set_device_time(unix_secs: u64) -> Vec<u8> {
     encode_frame(&data)
 }
 
+/// CMD_SET_RADIO_PARAMS (0x0B): set the LoRa PHY config. The device reboots to
+/// apply. `freq_field` and `bw_field` are the raw firmware fields (freq =
+/// MHz×1000 e.g. 869618 for 869.618 MHz; bw = kHz×1000 e.g. 62500 for 62.5 kHz);
+/// `sf` is 5..=12 and `cr` is 5..=8. Wire format verified against the MeshCore
+/// companion firmware handler (`examples/companion_radio/MyMesh.cpp`,
+/// `CMD_SET_RADIO_PARAMS`): `[11][freq:u32 LE][bw:u32 LE][sf:u8][cr:u8]`. The
+/// same fields (same units) come back in the SELF_INFO reply, so a caller can
+/// read them to detect drift. Values outside the firmware's accepted ranges are
+/// rejected by the device (it replies with an error frame), not clamped here.
+pub fn build_set_radio_params(freq_field: u32, bw_field: u32, sf: u8, cr: u8) -> Vec<u8> {
+    let mut data = vec![CMD_SET_RADIO_PARAMS];
+    data.extend_from_slice(&freq_field.to_le_bytes());
+    data.extend_from_slice(&bw_field.to_le_bytes());
+    data.push(sf);
+    data.push(cr);
+    encode_frame(&data)
+}
+
 /// CMD_SET_ADVERT_NAME (0x08): Set the node's advertised name on the mesh.
 pub fn build_set_advert_name(name: &str) -> Vec<u8> {
     let mut data = vec![CMD_SET_ADVERT_NAME];
@@ -730,6 +748,29 @@ mod tests {
         assert_eq!(frame[4], PROTOCOL_VERSION);
     }
 
+    #[test]
+    fn test_build_set_radio_params_wire_layout() {
+        // Portugal preset: 869.618 MHz, 62.5 kHz BW, SF 8, CR 8.
+        // freq field = MHz*1000 = 869618; bw field = kHz*1000 = 62500.
+        let frame = build_set_radio_params(869_618, 62_500, 8, 8);
+        assert_eq!(frame[0], OUTBOUND_MARKER);
+        // payload length = 1 (cmd) + 4 (freq) + 4 (bw) + 1 (sf) + 1 (cr) = 11
+        assert_eq!(u16::from_le_bytes([frame[1], frame[2]]), 11);
+        let data = &frame[3..];
+        assert_eq!(data[0], CMD_SET_RADIO_PARAMS);
+        assert_eq!(
+            u32::from_le_bytes([data[1], data[2], data[3], data[4]]),
+            869_618
+        );
+        assert_eq!(
+            u32::from_le_bytes([data[5], data[6], data[7], data[8]]),
+            62_500
+        );
+        assert_eq!(data[9], 8); // sf
+        assert_eq!(data[10], 8); // cr
+        assert_eq!(data.len(), 11);
+    }
+
     #[test]
     fn test_decode_frame_complete() -> Result<()> {
         // Simulate an inbound frame: < + len(2) + [RESP_OK]