v1.6.3: Fix LXMF wire format, add link-based delivery

Fix opportunistic LXMF wire format to match Python reference
(strip dest_hash from payload, carried by RNS packet header).
Compute messageId in packFull() matching Python LXMessage.pack().
Prepend dest_hash in onPacketReceived() for correct unpack.
Add opportunistic-first delivery with background link establishment.
Fix boot crash from RNS::Link default constructor in global scope.

src/config/Config.h

@@ -6,8 +6,8 @@
 
 #define RATDECK_VERSION_MAJOR  1
 #define RATDECK_VERSION_MINOR  6
-#define RATDECK_VERSION_PATCH  2
-#define RATDECK_VERSION_STRING "1.6.2"
+#define RATDECK_VERSION_PATCH  3
+#define RATDECK_VERSION_STRING "1.6.3"
 
 // --- Feature Flags ---
 #define HAS_DISPLAY     true

src/reticulum/LXMFManager.h

@@ -44,7 +44,7 @@ private:
     std::deque<LXMFMessage> _outQueue;
 
     // Outbound link state (opportunistic-first, link upgrades in background)
-    RNS::Link _outLink;
+    RNS::Link _outLink{RNS::Type::NONE};
     RNS::Bytes _outLinkDestHash;
     bool _outLinkPending = false;