JP_STRICT: dynamic MAX_TEXT_LEN based on runtime CR value

Instead of compile-time LORA_CR define, MAX_TEXT_LEN is now determined
at runtime by reading the actual coding rate from the radio hardware.

Added getMaxTextLen() to RadioLibWrapper and Dispatcher:
- CR4/5: 48 bytes (~16 JP chars, TX ~3808ms)
- CR4/6: 32 bytes (~10 JP chars)
- CR4/7: 24 bytes (~8 JP chars)
- CR4/8: 16 bytes (~5 JP chars, default)

getCodingRate() added to CustomSX1262Wrapper to read codingRate
from RadioLib PhysicalLayer at runtime.

Tested: 48-byte limit with LORA_CR=5, 16-byte limit with LORA_CR=8.

src/Dispatcher.h

@@ -38,6 +38,10 @@ public:
 
   virtual float packetScore(float snr, int packet_len) = 0;
 
+#ifdef JP_STRICT
+  virtual int getMaxTextLen() const { return 1 * 16; }  // default: CR4/8
+#endif
+
   /**
    * \brief  starts the raw packet send. (no wait)
    * \param  bytes   the raw packet data