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

examples/meshcore/meshcore_rx.py

@@ -916,7 +916,7 @@ RADIO_CONFIG = {
     "sf": 7,
     "cr": 5,  # CR 4/5 — denominator value
     "sync_word": 0x3444,
-    "tx_power_dbm": 14,
+    "tx_power_dbm": -128,  # TX_POWER_MAX: use board's maximum
 }