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/terrain_editor.hpp

@@ -43,6 +43,11 @@ public:
 
     // Get chunks modified since last call (for re-upload)
     std::vector<int> consumeDirtyChunks();
+    void markDirty(int chunkIdx) { dirtyChunks_.push_back(chunkIdx); dirty_ = true; }
+    void stitchChunkEdges(int chunkIdx) { stitchEdges(chunkIdx); }
+    glm::vec3 getChunkVertexWorldPos(int ci, int vi) const { return chunkVertexWorldPos(ci, vi); }
+    void beginGeneratorUndo() { recordGeneratorUndo(); }
+    void endGeneratorUndo() { commitGeneratorUndo(); }
 
     // Regenerate mesh for specific chunks
     pipeline::TerrainMesh regenerateMesh() const;