Version 0.9.0 - New Vertex Editing Mode

New Feature: Vertex Edit Mode

1. Convert any FLAT_SQUARE tile into a vertex-editable tile with 4 draggable RED corner handles
2. Drag corners freely in 3D to create slopes, ramps, deformed surfaces, and organic shapes
3. All corner positions snap to half-grid (grid_size / 2) for precise placement
4. Backfaces render as RED so you can always see which side is the front face
5. Full undo/redo support for convert, delete, and every corner drag

How to use Vertex Edit Mode:

1. Switch to Vertex Edit mode in the main toolbar
2. Click tiles to highlight them (cyan glow) — multiple tiles can be selected
3. Click the Convert button in the context toolbar to convert highlighted tiles
4. Click a converted tile to select it for editing — RED corner handles appear
5. Drag any corner handle to reshape the tile in 3D
6. Press DELETE to remove selected vertex tiles