feat(editor): flatten around object, scatter auto-align, manifest batch

- Flatten Ground: flattens terrain to object height with smooth falloff
  around placed buildings/structures (undoable). Button in object panel
- Scatter auto-align: checkbox enables terrain snapping + slope alignment
  for scattered objects (trees snap to ground and lean with hillsides)
- Batch convert now falls back to asset manifest when filesystem dir is
  empty — converts M2/WMO from game data without filesystem extraction
- Public terrain editor wrappers: beginGeneratorUndo, endGeneratorUndo,
  markDirty, stitchChunkEdges, getChunkVertexWorldPos

tools/editor/editor_app.hpp

@@ -90,6 +90,7 @@ public:
     void setSkyPreset(int preset); // 0=day, 1=dusk, 2=night
     void snapSelectedToGround();
     void alignSelectedToTerrain();
+    void flattenAroundSelected(float radius = 30.0f);
     void flyToSelected();
     int batchConvertAssets(const std::string& dataDir);
     void clearAllObjects();