feat(llcc68): make payload limit configurable

Add CONFIG_LLCC68_MAX_PAYLOAD_LENGTH with a default of 128 bytes and a hardware-bounded range of 1..255.

Wire both the C++ and raw C LLCC68 payload buffer constants to the Kconfig value so application builds can tune radio buffer RAM without editing headers.

Rename the fixed-length 100k preset from GMSK to GFSK and expose rx_bandwidth_hz() next to the GfskRxBandwidth enum so applications can report configured bandwidth without carrying driver-specific lookup tables.

include/llcc68_definitions.hpp

@@ -376,6 +376,55 @@ enum class GfskRxBandwidth : uint8_t {
   Bw467000 = 0x09,
 };
 
+inline uint32_t rx_bandwidth_hz(GfskRxBandwidth bandwidth) {
+  using enum GfskRxBandwidth;
+  switch (bandwidth) {
+  case Bw4800:
+    return 4800;
+  case Bw5800:
+    return 5800;
+  case Bw7300:
+    return 7300;
+  case Bw9700:
+    return 9700;
+  case Bw11700:
+    return 11700;
+  case Bw14600:
+    return 14600;
+  case Bw19500:
+    return 19500;
+  case Bw23400:
+    return 23400;
+  case Bw29300:
+    return 29300;
+  case Bw39000:
+    return 39000;
+  case Bw46900:
+    return 46900;
+  case Bw58600:
+    return 58600;
+  case Bw78200:
+    return 78200;
+  case Bw93800:
+    return 93800;
+  case Bw117300:
+    return 117300;
+  case Bw156200:
+    return 156200;
+  case Bw187200:
+    return 187200;
+  case Bw234300:
+    return 234300;
+  case Bw312000:
+    return 312000;
+  case Bw373600:
+    return 373600;
+  case Bw467000:
+    return 467000;
+  }
+  return 0;
+}
+
 enum class GfskPreambleDetector : uint8_t {
   Off = 0x00,
   Bits8 = 0x04,
@@ -478,7 +527,7 @@ struct gfsk_parameters_t {
   }
 
   static constexpr gfsk_parameters_t
-  Gmsk100kFixed6(freq_t frequency_mhz = 434.18,
+  Gfsk100kFixed6(freq_t frequency_mhz = 434.18,
                  tx_params_t tx_params = tx_params_t::Default()) {
     return {
         .mod_params =