feat(map-editor): v0.2.0b — bottom palette + Auto-Tile vertex paint

UI:
 - Bottom palette strip (above status, left of side-panel): 14
   coloured swatches numbered 1-14. Click selects state.active_tile.
 - World-space overlay (Auto-Tile mode only): cell grid + vertex
   dots + snapped-vertex highlight ring.
 - Side-panel + status repositioned to clear the palette band.
 - Cheatsheet updated with the new LMB/RMB canvas + palette rows.

Paint:
 - Auto-Tile mode: LMB on canvas -> maps.set_vertex(active_layer,
   snap.vx, snap.vy, true); RMB -> set_vertex(..., false). Snap is
   computed in update() by rounding world coords to the nearest
   (W+1)x(H+1) vertex; off-map cursor publishes nil.
 - Direct mode: unchanged from 0.2.0a (paint_cell_at with active
   slot + rotation).

Notes:
 - Palette swatches are placeholder solid colours, not real tile
   thumbnails — that needs a tile-UV getter on lib-core.maps, which
   is out of scope for this sub-slice.
 - Transform controls, Direct-Override transform writes, and the
   Decal/Entity sub-selectors land in 0.2.0c/d.

manifest.module

@@ -1,6 +1,6 @@
 {
     "id": "map-editor",
-    "version": "0.2.0a",
+    "version": "0.2.0b",
     "kind": "module",
     "api": "^0.1",
     "ci_frames": 30,