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/dComponents/GhostComponent.cpp

@@ -68,7 +68,9 @@ void GhostComponent::GhostEntity(LWOOBJID id) {
 	m_ObservedEntities.erase(id);
 }
 
-bool GhostComponent::OnToggleGMInvis(GameMessages::ToggleGMInvis& gmInvisMsg) {
+bool GhostComponent::OnToggleGMInvis(GameMessages::ToggleGMInvisEvent& event) {
+	auto& gmInvisMsg = event.msg;
+	gmInvisMsg.target = event.target; // the wire message goes out with the target the event was delivered to
 	gmInvisMsg.bStateOut = !m_IsGMInvisible;
 	m_IsGMInvisible = !m_IsGMInvisible;
 	LOG_DEBUG("GM Invisibility toggled to: %s", m_IsGMInvisible ? "true" : "false");