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/DestroyableComponent.cpp

@@ -783,7 +783,7 @@ void DestroyableComponent::Smash(const LWOOBJID source, const eKillType killType
 				lootMsg.spawnPos = m_Parent->GetPosition();
 				lootMsg.sourceID = source;
 				lootMsg.item = LOT_NULL;
-				lootMsg.Send();
+				GameMessages::DeliverLocally(lootMsg);
 				character->SetCoins(coinsTotal, eLootSourceType::DELETION);
 			}
 		}
@@ -1022,17 +1022,17 @@ void DestroyableComponent::DoHardcoreModeDrops(const LWOOBJID source) {
 			lootMsg.spawnPos = m_Parent->GetPosition();
 			lootMsg.sourceID = source;
 			lootMsg.item = LOT_NULL;
-			lootMsg.Send();
+			GameMessages::DeliverLocally(lootMsg);
 			lootMsg.Send(m_Parent->GetSystemAddress());
 			while (coinsToDrop > MAX_TO_DROP_PER_GM) {
 				LOG("Dropping 100,000, %llu left", coinsToDrop);
 				lootMsg.currency = 100'000;
-				lootMsg.Send();
+				GameMessages::DeliverLocally(lootMsg);
 				lootMsg.Send(m_Parent->GetSystemAddress());
 				coinsToDrop -= 100'000;
 			}
 			lootMsg.currency = coinsToDrop;
-			lootMsg.Send();
+			GameMessages::DeliverLocally(lootMsg);
 			lootMsg.Send(m_Parent->GetSystemAddress());
 		}
 		return;