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/dUtilities/Loot.cpp

@@ -136,7 +136,7 @@ void DropFactionLoot(Entity& player, GameMessages::DropClientLoot& lootMsg) {
 		lootMsg.lootID = ObjectIDManager::GenerateObjectID();
 		CalcFinalDropPos(lootMsg);
 		// Register the drop on the player
-		lootMsg.Send();
+		GameMessages::DeliverLocally(lootMsg);
 		// Visually drop it for the player
 		lootMsg.Send(player.GetSystemAddress());
 	}
@@ -165,7 +165,7 @@ void DropFactionLoot(const Team& team, GameMessages::DropClientLoot& lootMsg, co
 			lootMsg.lootID = ObjectIDManager::GenerateObjectID();
 			CalcFinalDropPos(lootMsg);
 			// Register the drop on this team member
-			lootMsg.Send();
+			GameMessages::DeliverLocally(lootMsg);
 			// Show the rewards on all connected members of the team. Only the loot owner will be able to pick the tokens up.
 			DistrbuteMsgToTeam(lootMsg, team);
 		}
@@ -184,7 +184,7 @@ void DropPowerupLoot(Entity& player, GameMessages::DropClientLoot& lootMsg) {
 	lootMsg.target = playerID;
 
 	// Register the drop on the player
-	lootMsg.Send();
+	GameMessages::DeliverLocally(lootMsg);
 	// Visually drop it for the player
 	lootMsg.Send(player.GetSystemAddress());
 }
@@ -210,7 +210,7 @@ void DropPowerupLoot(const Team& team, GameMessages::DropClientLoot& lootMsg, co
 
 		lootMsg.target = member;
 		// By sending this message with the same ID to all players on the team, all players on the team are allowed to pick it up.
-		lootMsg.Send();
+		GameMessages::DeliverLocally(lootMsg);
 		// No need to send to all members in a loop since that will happen by using the outer loop above and also since there is no owner
 		// sending to all will do nothing.
 		const auto* const memberEntity = Game::entityManager->GetEntity(member);
@@ -232,7 +232,7 @@ void DropMissionLoot(Entity& player, GameMessages::DropClientLoot& lootMsg) {
 		lootMsg.lootID = ObjectIDManager::GenerateObjectID();
 		CalcFinalDropPos(lootMsg);
 		// Register the drop with the player
-		lootMsg.Send();
+		GameMessages::DeliverLocally(lootMsg);
 		// Visually drop the loot to be picked up
 		lootMsg.Send(player.GetSystemAddress());
 	}
@@ -261,7 +261,7 @@ void DropMissionLoot(const Team& team, GameMessages::DropClientLoot& lootMsg, co
 			lootMsg.lootID = ObjectIDManager::GenerateObjectID();
 			CalcFinalDropPos(lootMsg);
 			// Register the drop with the player
-			lootMsg.Send();
+			GameMessages::DeliverLocally(lootMsg);
 			DistrbuteMsgToTeam(lootMsg, team);
 		}
 	}
@@ -279,7 +279,7 @@ void DropRegularLoot(Entity& player, GameMessages::DropClientLoot& lootMsg) {
 	lootMsg.target = playerID;
 	lootMsg.ownerID = playerID;
 	// Register the drop with the player
-	lootMsg.Send();
+	GameMessages::DeliverLocally(lootMsg);
 	// Visually drop the loot to be picked up
 	lootMsg.Send(player.GetSystemAddress());
 
@@ -309,14 +309,14 @@ void DropRegularLoot(Team& team, GameMessages::DropClientLoot& lootMsg, const bo
 	if (team.lootOption == 0 /* Shared loot */) {
 		lootMsg.target = earningPlayer;
 		lootMsg.ownerID = earningPlayer;
-		lootMsg.Send();
+		GameMessages::DeliverLocally(lootMsg);
 	} else /* Free for all loot */ {
 		lootMsg.ownerID = LWOOBJID_EMPTY;
 		// By sending the loot with NO owner and to ALL members of the team,
 		// its a first come, first serve with who picks the item up.
 		for (const auto ffaMember : team.members) {
 			lootMsg.target = ffaMember;
-			lootMsg.Send();
+			GameMessages::DeliverLocally(lootMsg);
 		}
 	}
 
@@ -405,7 +405,7 @@ void DropLoot(Entity* player, const LWOOBJID source, const std::map<LOT, LootDro
 		lootMsg.sourceID = source;
 		lootMsg.item = LOT_NULL;
 		CalcFinalDropPos(lootMsg);
-		lootMsg.Send();
+		GameMessages::DeliverLocally(lootMsg);
 		const auto* const memberEntity = Game::entityManager->GetEntity(member);
 		if (memberEntity) lootMsg.Send(memberEntity->GetSystemAddress());
 	}