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/board/heltec_v3.rs

@@ -25,6 +25,9 @@ pub type UsbDriver = esp_hal::otg_fs::asynch::Driver<'static>;
 
 pub type DisplayI2c = I2c<'static, esp_hal::Async>;
 
+/// TX power range for this board's radio (SX1262).
+pub const TX_POWER_RANGE: (i8, i8) = (-9, 22);
+
 // ── Peripheral bundles ───────────────────────────────────────────────
 
 pub struct RadioParts {

src/board/heltec_v4.rs

@@ -25,6 +25,9 @@ pub type UsbDriver = esp_hal::otg_fs::asynch::Driver<'static>;
 
 pub type DisplayI2c = I2c<'static, esp_hal::Async>;
 
+/// TX power range for this board's radio (SX1262).
+pub const TX_POWER_RANGE: (i8, i8) = (-9, 22);
+
 // ── Peripheral bundles ───────────────────────────────────────────────
 
 pub struct RadioParts {

src/board/rak_wisblock_4631.rs

@@ -33,6 +33,9 @@ pub type UsbDriver = Driver<'static, USBD, &'static embassy_nrf::usb::vbus_detec
 /// I2C bus for an optional SSD1306 OLED.
 pub type DisplayI2c = Twim<'static, embassy_nrf::peripherals::TWISPI0>;
 
+/// TX power range for this board's radio (SX1262).
+pub const TX_POWER_RANGE: (i8, i8) = (-9, 22);
+
 // ── Peripheral bundles ───────────────────────────────────────────────
 
 pub struct RadioParts {