fix(power): use XOSC standby to keep TCXO on through CAD→RX; add burst window

- cadWake() now sets standbyXOSC=true so RadioLib's internal standby() calls
  keep the TCXO powered continuously through the CAD scan and into startReceive,
  eliminating the freq-drift window that dropped ~50% of long packets
- PS_RX_TIMEOUT_MS 1500→3000 to cover 256 B packets at SF8/BW62.5/CR5 (~1925 ms)
- 500 ms burst window after each successful receive: skip inter-scan sleep so
  back-to-back messages are caught without waiting out the 65 ms cadence

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/helpers/radiolib/CustomSX1262Wrapper.h

@@ -41,14 +41,15 @@ public:
   void doResetAGC() override { sx126xResetAGC((SX126x *)_radio); }
 
   // Warm sleep between CAD scans: ~1.5 µA vs ~600 µA in standby-RC.
-  // cadWake() delays 5 ms so the TCXO fully re-stabilises before the next scan
-  // (previous attempt without the delay corrupted long packets via freq drift).
+  // cadWake() sets standbyXOSC=true so every subsequent internal standby() call
+  // keeps the TCXO powered — no gap between CAD and startReceive, no freq drift.
+  // The chip's programmed TCXO timeout (set by begin()) handles stabilisation.
   void cadSleep() override {
     ((CustomSX1262 *)_radio)->sleep(true);  // warm sleep — config retained
   }
   void cadWake() override {
-    _radio->standby();
-    delayMicroseconds(5000);  // TCXO stabilisation after warm sleep
+    ((SX126x *)_radio)->standbyXOSC = true;  // all subsequent standby() → XOSC
+    _radio->standby();                         // wake into standby-XOSC, TCXO on
   }
 
   void setRxBoostedGainMode(bool en) override {

src/helpers/radiolib/RadioLibWrappers.cpp

@@ -24,8 +24,13 @@
 // → 65 ms gives 2 guaranteed windows with margin. Revisit if SF/BW change.
 #define PS_CAD_INTERVAL_MS  65
 // If CAD detects activity but no packet actually lands, fall back to scanning
-// instead of sitting in continuous RX forever.
-#define PS_RX_TIMEOUT_MS    1500
+// instead of sitting in continuous RX forever. Must exceed the longest possible
+// packet air-time: 256 B at SF8/BW62.5/CR5 ≈ 1925 ms from preamble start, so
+// 3000 ms gives ~1 s margin for a false-detect case.
+#define PS_RX_TIMEOUT_MS    3000
+// After a successful receive, skip inter-scan sleep for this long so back-to-back
+// messages are caught without waiting out the 65 ms window each time.
+#define PS_BURST_WINDOW_MS  500
 
 static volatile uint8_t state = STATE_IDLE;
 
@@ -160,6 +165,10 @@ void RadioLibWrapper::armRecv() {
 }
 
 void RadioLibWrapper::enterCadSleep() {
+  if ((int32_t)(millis() - _ps_burst_until) < 0) {
+    armRecv();   // burst window: skip sleep, scan again immediately
+    return;
+  }
   cadSleep();                     // warm sleep (SX126x) or standby fallback between scans
   _ps_phase = PS_SLEEPING;
   _ps_timer = millis() + PS_CAD_INTERVAL_MS;
@@ -228,6 +237,7 @@ int RadioLibWrapper::recvRaw(uint8_t* bytes, int sz) {
       } else {
       //  Serial.print("  readData() -> "); Serial.println(len);
         n_recv++;
+        _ps_burst_until = millis() + PS_BURST_WINDOW_MS;
       }
     }
     state = STATE_IDLE;   // need another startReceive()

src/helpers/radiolib/RadioLibWrappers.h

@@ -27,6 +27,7 @@ protected:
   bool     _power_save = false;
   uint8_t  _ps_phase = 0;        // PS_* (see .cpp): scanning / sleeping / rxing
   uint32_t _ps_timer = 0;        // next-scan time, or RX false-detect deadline
+  uint32_t _ps_burst_until = 0;  // scan continuously until this time after RX
   void armRecv();                // arm RX honouring power-save (CAD) or continuous
   void powerSaveLoop();
   void enterCadSleep();