SX126x: re-apply 0x8B5 register in resetAGC() to preserve RX sensitivity (#10219)

The CALIBRATE_ALL (0x7F) command inside resetAGC() clears bit 0 of the
undocumented 0x8B5 register. That bit is set once in init() by #9571 and
#9777 to improve SX1262 RX sensitivity, and the AGC-reset path was not
re-applying it. Result: every SX1262 node silently loses the RX
sensitivity patch ~60s after boot and never recovers until reboot.

Empirically confirmed on Heltec Mesh Node T114 (nRF52840 + SX1262):
  - Post-calibration read of 0x8B5 = 0x04 (bit 0 cleared)
  - After re-apply: 0x05 (bit 0 set)
Reproducible every AGC_RESET_INTERVAL_MS tick.

Fix re-applies the register bit alongside the existing post-calibration
re-applies (setDio2AsRfSwitch, setRxBoostedGainMode).

src/mesh/SX126xInterface.cpp

@@ -455,6 +455,15 @@ template <typename T> void SX126xInterface<T>::resetAGC()
     // RX boosted gain mode
     lora.setRxBoostedGainMode(config.lora.sx126x_rx_boosted_gain);
 
+    // Re-apply the undocumented 0x8B5 RX sensitivity patch that was set in init().
+    // The CALIBRATE_ALL (0x7F) command above clears bit 0 of register 0x8B5, which
+    // silently removes the RX sensitivity improvement introduced in #9571 / #9777.
+    // Without this re-apply, every SX1262 node loses its RX boost ~60s after boot
+    // and never recovers until reboot. See empirical evidence in the PR description.
+    if (module.SPIsetRegValue(0x8B5, 0x01, 0, 0) != RADIOLIB_ERR_NONE) {
+        LOG_WARN("SX126x resetAGC: failed to re-apply 0x8B5 RX sensitivity patch");
+    }
+
     // 6. Resume receiving
     startReceive();
 }