lxmf: avoid split LoRa opportunistic sends

Gate rsDeck opportunistic LXMF sends by the final packed Reticulum raw size when the next hop is LoRa, instead of using the pre-encryption LXMF payload size as the 254-byte limit.

Keep TCP paths on the normal Reticulum MDU, route LoRa packets that would exceed one RNode RF frame into link delivery, and log payload/raw/lora_raw sizing for field diagnosis.

Tested with rsDeck/Ratdeck T-Deck and Heltec V3 RNode attached over USB to a MacBook Pro running Ratspeak, using the medium-fast LoRa preset. Short 1-5 character opportunistic sends worked bidirectionally. Direct link delivery worked from T-Deck to Ratspeak/Heltec; Ratspeak-to-T-Deck link delivery later queued during link establishment while opportunistic still worked.

src/main.cpp

@@ -711,7 +711,7 @@ static constexpr uint8_t LITE_TRANSPORT_ID[16] = {
     'r', 's', 'l', 'i', 't', 'e', '-', 'h',
     'e', 'l', 't', 'e', 'c', '-', 'v', '3'
 };
-static constexpr size_t RNODE_DIAG_SINGLE_MTU = 254;
+static constexpr size_t RNODE_DIAG_SINGLE_MTU = RSDECK_RNODE_SINGLE_FRAME_RAW_MAX;
 static RNS::Bytes diagnosticLiteLinkId;
 
 static bool sendDiagnosticRawReticulum(const RNS::Bytes& raw, const char* label) {