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_app.hpp

@@ -76,6 +76,7 @@ public:
     void setSkyPreset(int preset); // 0=day, 1=dusk, 2=night
     void snapSelectedToGround();
     void flyToSelected();
+    void clearAllObjects();
     void centerOnTerrain();
 
     // Multi-tile support