refactor: activity game messages as structs

Converts ActivityEnter, ActivityExit, ActivityStart, ActivityStop,
ActivityPause, StartActivityTime, RequestActivityEnter,
RequestActivityExit and ShowActivityCountdown to NetGameMsgs in
ActivityMessages.{h,cpp}, switches their callers (racing, shooting
gallery, survival, waves, AG course, NT combat challenge) and deletes
the old GameMessages::Send* / HandleRequestActivityExit functions.
REQUEST_ACTIVITY_EXIT is registered in GameMessageHandler's map.

No wire change. Verified by comparing every struct byte for byte with a
frozen verbatim copy of the old functions (tests/.../Legacy) over a
grid of inputs, both to one client and as a broadcast (a temporary test
proved the copy matched production before it was deleted), plus hand
computed golden bytes, round trips and a deliberate field mutation that
made the tests fail. Layouts confirmed against the 1.10.64 client in
Ghidra. Only difference: a broadcast no longer makes
the extra Send(UNASSIGNED, false) that RakNet already rejected.

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

dGame/dComponents/RacingControlComponent.cpp

@@ -8,6 +8,7 @@
 #include "DestroyableComponent.h"
 #include "EntityManager.h"
 #include "GameMessages.h"
+#include "ActivityMessages.h"
 #include "InventoryComponent.h"
 #include "Item.h"
 #include "MissionComponent.h"
@@ -700,8 +701,9 @@ void RacingControlComponent::Update(float deltaTime) {
 				}
 
 				// Start the race
-				GameMessages::SendActivityStart(m_Parent->GetObjectID(),
-					UNASSIGNED_SYSTEM_ADDRESS);
+				GameMessages::ActivityStart activityStart;
+				activityStart.target = m_Parent->GetObjectID();
+				activityStart.Send(UNASSIGNED_SYSTEM_ADDRESS);
 
 				m_Started = true;