Prevent forced TX during JP LBT backoff via getCADFailMaxDuration()

Add isJapanMode() virtual to mesh::Radio (default false) so Dispatcher
can query frequency context without depending on RadioLibWrapper.
Dispatcher::getCADFailMaxDuration() returns UINT32_MAX for JP nodes,
eliminating the 4-second forced-TX that would violate ARIB STD-T108.
Non-JP nodes retain the original 4-second safety timeout unchanged.

src/Dispatcher.cpp

@@ -60,6 +60,7 @@ uint32_t Dispatcher::getCADFailRetryDelay() const {
   return 200;
 }
 uint32_t Dispatcher::getCADFailMaxDuration() const {
+  if (_radio->isJapanMode()) return UINT32_MAX;  // ARIB STD-T108: never force TX during LBT
   return 4000;   // 4 seconds
 }
 

src/Dispatcher.h

@@ -78,6 +78,8 @@ public:
 
   virtual float getLastRSSI() const { return 0; }
   virtual float getLastSNR() const { return 0; }
+
+  virtual bool isJapanMode() const { return false; }
 };
 
 /**

src/helpers/radiolib/RadioLibWrappers.h

@@ -57,7 +57,7 @@ public:
   virtual uint8_t getCodingRate() const { return 8; }   // default CR4/8, override in subclass
   virtual float getFreqMHz() const { return 0.0f; }     // default unknown, override in subclass
 
-  bool isJapanMode() const {
+  bool isJapanMode() const override {
     float freq = getFreqMHz();
     return (fabsf(freq - 920.800f) < 0.05f ||
             fabsf(freq - 921.000f) < 0.05f ||