feat: scene ghosting, as the client streams a zone's scenes

The client keeps the scene under the player loaded, the scenes the zone
file's transitions connect to it, and the global scene
(Zone::StreamScenesAroundPosition 0x0108a3f0, TerrainManager::GetSceneAtPos
0x01069010, the connected scenes at 0x01066500; transitions naming a
missing scene dropped as Zone::FixupInvalidTransitions 0x010842e0 does).

- ZoneScenes (dCommon): the terrain's scene map lookup and the scene graph,
  shared by the world server and the dashboard.
- Objects remember the scene they were placed in (spawners pass theirs on).
- ghosting_scenes=1 (world config, off by default): players get the objects
  of their loaded scenes instead of the ones within the ghosting distances;
  objects from no scene go by the scene under them. Zones without a scene
  map keep distance ghosting.

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

tests/dGameTests/SceneGhostingTests.cpp

@@ -0,0 +1,27 @@
+#include "GameDependencies.h"
+#include <gtest/gtest.h>
+
+#include "Entity.h"
+#include "ZoneScenes.h"
+
+class SceneGhostingTest : public GameDependenciesTest {
+protected:
+	void SetUp() override { SetUpDependencies(); }
+	void TearDown() override { TearDownDependencies(); }
+};
+
+// Objects keep the scene they were placed in, which scene ghosting goes by; objects from no scene go by their position
+TEST_F(SceneGhostingTest, EntitiesKeepTheirScene) {
+	EntityInfo placed = info;
+	placed.scene = 12;
+	const auto fromScene = std::make_unique<Entity>(20, placed);
+	EXPECT_EQ(fromScene->GetScene(), 12);
+	const auto spawned = std::make_unique<Entity>(21, info);
+	EXPECT_EQ(spawned->GetScene(), -1);
+
+	// A player standing in scene 3, whose client keeps 0, 3 and 5 loaded
+	const std::set<uint32_t> loaded{ 0, 3, 5 };
+	EXPECT_FALSE(ZoneScenes::InLoadedScene(fromScene->GetScene(), 3, loaded)); // placed in scene 12, though it stands in 3
+	EXPECT_TRUE(ZoneScenes::InLoadedScene(spawned->GetScene(), 5, loaded));
+	EXPECT_FALSE(ZoneScenes::InLoadedScene(spawned->GetScene(), 12, loaded));
+}