chore: foundations for struct based packets and game messages

First step of moving hand written bitstream code to struct based
messages (see docs/PacketArchitecture.md). No wire changes.

- GameMsg is now only a server-internal event (delivered to handlers
  registered with RegisterMsg); NetGameMsg is a wire message with
  Send(sysAddr) (UNASSIGNED broadcasts), SendToClient(sysAddr) (one
  client, never broadcasts), WritePacket, Serialize, Deserialize and
  Handle. Mixing them up is now a compile error. NetGameMsgEvent<T> /
  DeliverLocally carry a wire message to local handlers; loot drops,
  pickup, the object debugger, GM invisibility and model RequestUse
  use them. The GM invisibility message keeps its target, so its bytes
  are unchanged.
- Behaviour change: GameMessageHandler logs and drops messages whose
  Deserialize fails instead of handling them with default fields (the
  4 messages already registered: RequestUse, RequestServerObjectInfo,
  ShootingGalleryFire, PickupItem).
- LUBitStream (kept as on main) gets a virtual destructor (Mail deleted
  derived packets through the base) and WritePacket, also used by
  ChatPackets::SendRoutedMsg with identical bytes.
- BitStreamUtils::WriteOptional/ReadOptional for default-flag fields
  and WriteLengthPrefixed/ReadLengthPrefixed for length prefixed
  strings.
- Every message ID enumerator (MessageType::*, ServiceType, Mail's
  wire enums) is pinned with static_asserts, so renumbering or removing
  one fails to compile.
- Tests: dServerMock copies each sent packet (it kept a pointer to the
  caller's destroyed BitStream); PacketTestUtils.h compares packets bit
  for bit; helper tests use hand computed golden bytes and equality
  with the hand written patterns they replace; compile-time checks
  keep the wire/internal split in place.
- docs/PacketArchitecture.md: survey, target architecture,
  conventions, verification method and migration plan.

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

dGame/dGameMessages/GameMessages.cpp

@@ -6285,14 +6285,24 @@ namespace GameMessages {
 		return Send();
 	}
 
-	void GameMsg::Send(const SystemAddress& sysAddr) const {
-		CBITSTREAM;
+	void NetGameMsg::WritePacket(RakNet::BitStream& bitStream) const {
 		CMSGHEADER;
 
 		bitStream.Write(target); // Who this message will be sent to on the (a) client
 		bitStream.Write(msgId); // the ID of this message
 
 		Serialize(bitStream); // write the message data
+	}
+
+	void NetGameMsg::SendToClient(const SystemAddress& sysAddr) const {
+		CBITSTREAM;
+		WritePacket(bitStream);
+		SEND_PACKET;
+	}
+
+	void NetGameMsg::Send(const SystemAddress& sysAddr) const {
+		CBITSTREAM;
+		WritePacket(bitStream);
 
 		// Send to everyone if someone sent unassigned system address, or to one specific client.
 		if (sysAddr == UNASSIGNED_SYSTEM_ADDRESS) {
@@ -6378,8 +6388,10 @@ namespace GameMessages {
 
 	void RequestServerObjectInfo::Handle(Entity& entity, const SystemAddress& sysAddr) {
 		auto* handlingEntity = Game::entityManager->GetEntity(targetForReport);
-		if (handlingEntity) handlingEntity->HandleMsg(*this);
-		else LOG("Failed to find target %llu", targetForReport);
+		if (handlingEntity) {
+			RequestServerObjectInfoEvent event(*this);
+			handlingEntity->HandleMsg(event);
+		} else LOG("Failed to find target %llu", targetForReport);
 	}
 
 	bool RequestUse::Deserialize(RakNet::BitStream& stream) {
@@ -6418,7 +6430,8 @@ namespace GameMessages {
 			interactedObject->OnUse(&entity);
 		}
 
-		interactedObject->HandleMsg(*this);
+		RequestUseEvent event(*this);
+		interactedObject->HandleMsg(event);
 
 		//Perform use task if possible:
 		auto missionComponent = entity.GetComponent<MissionComponent>();
@@ -6480,7 +6493,8 @@ namespace GameMessages {
 		LOG("Has team %i picking up %llu:%llu", team != nullptr, lootID, lootOwnerID);
 		if (team) {
 			for (const auto memberId : team->members) {
-				this->Send(memberId);
+				PickupItemEvent event(*this);
+				event.Send(memberId);
 				TeamPickupItem teamPickupMsg{};
 				teamPickupMsg.target = lootID;
 				teamPickupMsg.lootID = lootID;