refactor: add Packet::hasData(), replace 52 position checks and 14 more Lua guards

Add Packet::hasData() for 'has remaining data' checks, replacing 52
verbose getReadPos()<getSize() comparisons across 3 files. Also replace
14 more standalone Lua return patterns with luaReturnNil/Zero helpers.

include/network/packet.hpp

@@ -46,6 +46,7 @@ public:
         return getRemainingSize() >= guidBytes;
     }
     void setReadPos(size_t pos) { readPos = pos; }
+    bool hasData() const { return readPos < data.size(); }
     void skipAll() { readPos = data.size(); }
 
 private: