JP LBT: make busy-channel backoff non-blocking

isChannelActive() previously spin-waited (with YIELD_TASK()) for up to
16s after detecting RSSI busy, blocking Dispatcher::loop() from
servicing incoming packets for the duration of the backoff.

Replace the blocking wait with a state machine (_lbt_backoff_active /
_lbt_deadline): on busy detection, arm a deadline and return true
(busy) immediately. Dispatcher::checkSend()'s existing retry mechanism
(getCADFailRetryDelay()) re-polls isChannelActive() on the next
loop() pass; once the deadline has elapsed, re-sense (bounded,
single-level recursion) instead of assuming still-busy.

The initial 5ms RSSI sense and the post-clear jitter wait remain
blocking, as both are short (<=5ms and <=~1s respectively) relative
to the up-to-16s backoff this change targets.

Hardware-tested (LilyGo T3S3 sx1262 / Wio Tracker L1 Pro) in both quiet and
electrically noisy environments; confirmed both bidirectional and
unidirectional message delivery under legitimate busy-channel
conditions.

src/helpers/radiolib/RadioLibWrappers.cpp

@@ -194,6 +194,18 @@ int16_t RadioLibWrapper::performChannelScan() {
 
 bool RadioLibWrapper::isChannelActive() {
   if (isAS923_1_JP()) {
+    // Non-blocking backoff: if a prior busy detection armed a backoff wait,
+    // report busy and let Dispatcher::checkSend() re-poll on the next loop()
+    // pass instead of spin-waiting here (which used to block the Dispatcher
+    // loop for up to 16s).
+    if (_lbt_backoff_active) {
+      if ((int32_t)(millis() - _lbt_deadline) < 0) {
+        return true;  // still waiting; re-checked next call
+      }
+      _lbt_backoff_active = false;
+      return isChannelActive();  // backoff elapsed -- re-sense (bounded, single-level recursion)
+    }
+
     // ARIB STD-T108: 5ms continuous RSSI sensing, -80dBm absolute threshold
     uint32_t sense_start = millis();
     while (millis() - sense_start < 5) {
@@ -201,10 +213,8 @@ bool RadioLibWrapper::isChannelActive() {
         _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();
-        }
+        _lbt_deadline = millis() + random(max_backoff / 2, max_backoff);
+        _lbt_backoff_active = true;
         return true;
       }
       YIELD_TASK();

src/helpers/radiolib/RadioLibWrappers.h

@@ -20,6 +20,9 @@ protected:
   int32_t _floor_sample_sum;
   uint8_t _preamble_sf;
 
+  bool _lbt_backoff_active;   // non-blocking wait after RSSI busy detection (up to 16s)
+  uint32_t _lbt_deadline;
+
   void idle();
   void startRecv();
   float packetScoreInt(float snr, int sf, int packet_len);
@@ -27,7 +30,7 @@ protected:
   virtual void doResetAGC();
 
 public:
-  RadioLibWrapper(PhysicalLayer& radio, mesh::MainBoard& board) : _radio(&radio), _board(&board), _busy_count(0), _preamble_sf(0) { n_recv = n_sent = 0; }
+  RadioLibWrapper(PhysicalLayer& radio, mesh::MainBoard& board) : _radio(&radio), _board(&board), _busy_count(0), _preamble_sf(0), _lbt_backoff_active(false), _lbt_deadline(0) { n_recv = n_sent = 0; }
 
   void begin() override;
   virtual void powerOff() { _radio->sleep(); }