Add ARIB STD-T108 LBT path to isChannelActive()

In JP 920MHz band (CH25-27): 5ms continuous RSSI sensing at -80dBm
absolute threshold, exponential backoff (2000-16000ms) on busy, jitter
(0-500ms) on free, then falls through to CAD if enabled.
Non-JP path: existing relative-RSSI threshold + CAD unchanged.
Add YIELD_TASK() macro and _busy_count field for backoff tracking.

src/helpers/radiolib/RadioLibWrappers.cpp

@@ -2,6 +2,12 @@
 #define RADIOLIB_STATIC_ONLY 1
 #include "RadioLibWrappers.h"
 
+#ifdef NRF52_PLATFORM
+  #define YIELD_TASK() vTaskDelay(1)
+#else
+  #define YIELD_TASK() delay(1)
+#endif
+
 #define STATE_IDLE       0
 #define STATE_RX         1
 #define STATE_TX_WAIT    3
@@ -184,10 +190,34 @@ int16_t RadioLibWrapper::performChannelScan() {
 }
 
 bool RadioLibWrapper::isChannelActive() {
-  // int.thresh: RSSI-based interference detection (relative to noise floor)
+  if (isJapanMode()) {
-  if (_threshold != 0 && getCurrentRSSI() > _noise_floor + _threshold) return true;
+    // ARIB STD-T108: 5ms continuous RSSI sensing, -80dBm absolute threshold
+    uint32_t sense_start = millis();
+    while (millis() - sense_start < 5) {
+      if (getCurrentRSSI() > -80.0f) {
+        _busy_count++;
+        uint32_t base_ms = 2000;
+        uint32_t max_backoff = min(base_ms * (1u << _busy_count), (uint32_t)16000);
+        uint32_t backoff_until = millis() + random(max_backoff / 2, max_backoff);
+        while (millis() < backoff_until) {
+          YIELD_TASK();
+        }
+        return true;
+      }
+      YIELD_TASK();
+    }
+    _busy_count = 0;
+    uint32_t jitter_until = millis() + random(0, 500);
+    while (millis() < jitter_until) {
+      YIELD_TASK();
+    }
+    // JP RSSI sensing passed; fall through to CAD if enabled
+  } else {
+    // Non-JP: RSSI-based interference detection (relative to noise floor)
+    if (_threshold != 0 && getCurrentRSSI() > _noise_floor + _threshold) return true;
+  }
 
-  // cad: hardware channel activity detection
+  // hardware channel activity detection (JP and non-JP)
   if (_cad_enabled) {
     int16_t result = performChannelScan();
     // scanChannel() triggers DIO interrupt (CAD done) which sets STATE_INT_READY

src/helpers/radiolib/RadioLibWrappers.h

@@ -15,6 +15,7 @@ protected:
   uint32_t n_recv, n_sent, n_recv_errors;
   int16_t _noise_floor, _threshold;
   bool _cad_enabled;
+  uint8_t _busy_count;
   uint16_t _num_floor_samples;
   int32_t _floor_sample_sum;
   uint8_t _preamble_sf;
@@ -26,7 +27,7 @@ protected:
   virtual void doResetAGC();
 
 public:
-  RadioLibWrapper(PhysicalLayer& radio, mesh::MainBoard& board) : _radio(&radio), _board(&board), _preamble_sf(0) { n_recv = n_sent = 0; }
+  RadioLibWrapper(PhysicalLayer& radio, mesh::MainBoard& board) : _radio(&radio), _board(&board), _busy_count(0), _preamble_sf(0) { n_recv = n_sent = 0; }
 
   void begin() override;
   virtual void powerOff() { _radio->sleep(); }