feat(power): CAD-windowed receive (battery saver) — phase 1

Add an optional low-power RX mode: instead of listening continuously, the
radio periodically runs a Channel Activity Detection scan and sits in standby
between scans; on detected activity it drops into a full receive to catch the
packet. Cuts average RX current several-fold (the dominant draw on this node).

- RadioLibWrapper: CAD state machine (scan → sleep window → rx) driven from
  loop(), which runs before recvRaw each cycle. `state` stays STATE_RX through
  all phases so the dispatcher's not-in-RX watchdog never trips. Re-arm routes
  through armRecv() in both startRecv() and recvRaw(); toggling on/off switches
  mode safely at the next idle. Falls back to continuous RX if CAD is
  unsupported. setPowerSaving()/getPowerSaving() control it.
- Software CSMA is already disabled on this firmware (interference threshold 0),
  so duty-cycling doesn't affect listen-before-talk; airtime budget still gates
  TX, and isReceivingPacket() still defers TX while a packet is mid-air.
- Companion: new rx_powersave pref (schema 0xC0DE0008), Settings › Radio ›
  "Pwr save" toggle, applied at boot (MyMesh) and on change (UITask).

CAD window = 45 ms (< the 16-symbol preamble at SF8/BW62.5) and CAD params use
RadioLib defaults — both will want on-hardware tuning + a before/after current
measurement to confirm reception is intact.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

examples/companion_radio/MyMesh.cpp

@@ -1203,6 +1203,7 @@ void MyMesh::begin(bool has_display) {
   radio_driver.setParams(_prefs.freq, _prefs.bw, _prefs.sf, _prefs.cr);
   radio_driver.setTxPower(_prefs.tx_power_dbm);
   radio_driver.setRxBoostedGainMode(_prefs.rx_boosted_gain);
+  radio_driver.setPowerSaving(_prefs.rx_powersave);   // CAD-windowed RX (battery saver)
   MESH_DEBUG_PRINTLN("RX Boosted Gain Mode: %s",
                      radio_driver.getRxBoostedGainMode() ? "Enabled" : "Disabled");
 }