fix(editor): stop destructive M2 rebuild on every NPC click, fix Clear All

Root cause of GPU crashes (VK_ERROR_DEVICE_LOST): every NPC placement
triggered a full clear+reload of ALL M2 models. After several cycles
the GPU state corrupted, causing vertex explosions and device lost.

Fixes:
- NPC placement now only updates cheap marker geometry (no M2 reload)
- Full M2 rebuild only happens when object COUNT changes (not every click)
- clearAllObjects() properly resets viewport, placer, spawner, markers,
  and history in one call with vkDeviceWaitIdle fence
- New Terrain uses clearAllObjects() for consistent reset
- Clear All menu item calls clearAllObjects()
- M2 vertex validation: rejects models with NaN/infinite/extreme
  vertex positions before GPU upload (prevents vertex explosions)
- NPC marker building extracted to updateNpcMarkers() method
  (can be called independently without M2 rebuild)

tools/editor/editor_ui.cpp

@@ -103,12 +103,7 @@ void EditorUI::renderMenuBar(EditorApp& app) {
                 ImGui::EndMenu();
             }
             if (ImGui::MenuItem("Clear All Objects/NPCs", nullptr, false, app.hasTerrainLoaded())) {
-                app.getObjectPlacer().clearAll();
-                app.getNpcSpawner().clearSelection();
-                app.getNpcSpawner().getSpawns().clear();
-                app.getTerrainEditor().history().clear();
-                app.markObjectsDirty();
-                app.showToast("All objects and NPCs cleared");
+                app.clearAllObjects();
             }
             ImGui::Separator();
             if (ImGui::MenuItem("Quick Save", "Ctrl+S", false, app.hasTerrainLoaded()))