feat: per-board TX power range, TX_POWER_MAX sentinel (-128)

- Each board exports TX_POWER_RANGE constant (min, max dBm)
- RadioConfig::validate() uses the board's range, not hardcoded limits
- TX_POWER_MAX sentinel (-128 on the wire) means "use board's maximum"
  — resolved to the actual max before storing config
- GetConfig returns the resolved actual power, not the sentinel
- Default configs in examples use TX_POWER_MAX
- Protocol docs updated

src/radio/task.rs

@@ -24,11 +24,12 @@ use embassy_time::Delay;
 use lora_phy::mod_params::RadioError;
 use lora_phy::{LoRa, RxMode};
 
-use crate::board::{RadioDriver, RadioParts};
+use crate::board::{RadioDriver, RadioParts, TX_POWER_RANGE};
 use crate::channel::{CommandChannel, RadioState, RadioStatus, ResponseChannel, StatusWatch};
 use crate::protocol::{self, Command, ErrorCode, RadioConfig, Response};
 
 const MAX_PAYLOAD: usize = protocol::MAX_PAYLOAD;
+const POWER_RANGE: (i8, i8) = TX_POWER_RANGE;
 
 type Radio = LoRa<RadioDriver, Delay>;
 
@@ -173,13 +174,14 @@ async fn handle_cmd(
             }
         }
         Command::SetConfig(cfg) => {
-            if let Err(reason) = cfg.validate() {
+            if let Err(reason) = cfg.validate(POWER_RANGE) {
                 warn!("SetConfig rejected: {}", reason);
                 responses
                     .send(Response::Error(ErrorCode::InvalidConfig))
                     .await;
             } else {
-                state.config = Some(cfg);
+                // Resolve TX_POWER_MAX sentinel to board's actual max
+                state.config = Some(cfg.resolve_power(POWER_RANGE));
                 status.sender().send(state.clone());
                 responses.send(Response::Ok).await;
             }
@@ -215,9 +217,9 @@ async fn handle_cmd(
         }
         Command::DisplayOn | Command::DisplayOff => {}
         Command::Transmit { config, payload } => {
-            let tx_config = config.or(state.config);
+            let tx_config = config.map(|c| c.resolve_power(POWER_RANGE)).or(state.config);
             if let Some(cfg) = tx_config {
-                if let Err(reason) = cfg.validate() {
+                if let Err(reason) = cfg.validate(POWER_RANGE) {
                     warn!("TX config rejected: {}", reason);
                     responses
                         .send(Response::Error(ErrorCode::InvalidConfig))