feat(capture): decode every LU packet with its own struct

The capture viewer's LU packet fields come from each struct's Deserialize and a member list generated from the
struct definitions (tools/gen_game_message_fields.py -> dNet/PacketFields.inc), replacing the hand-written list of
25. Secret members are never listed.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

dNet/PacketDecoder.h

@@ -74,6 +74,9 @@ namespace PacketDecoder {
 
 	// How many (service, message) pairs have typed decoders, for tests
 	size_t RegisteredCount();
+
+	// Whether a (service, message) has a struct its fields are read with
+	bool HasFields(ServiceType service, uint32_t messageId);
 }
 
 #endif  //!__PACKETDECODER__H__