fix(shelf): tuck cubby cell dividers into the boards they meet

Cubby per-cell dividers landed exactly flush on the board below and under the
board above — coplanar faces that shimmer and read as merged into the shelf.
Extend each divider 1mm into both boards (centre unchanged) so it tucks under
the top board and onto the bottom board with a solid, seam-free join.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

packages/nodes/src/shelf/geometry.ts

@@ -350,7 +350,13 @@ function buildCubby(group: Group, node: ShelfNode, materials: ShelfSlotMaterials
       for (let c = 1; c < node.columns; c++) {
         const x = -innerWidth / 2 + c * colStep
         const divider = stampShelfSlot(
-          new Mesh(new BoxGeometry(node.thickness, dividerHeight, node.depth), materials.frame),
+          // Extend 1mm into the boards above + below (centre unchanged) so the
+          // divider tucks under the top board and onto the bottom board instead
+          // of meeting them on a coplanar face (which shimmers / reads merged).
+          new Mesh(
+            new BoxGeometry(node.thickness, dividerHeight + 2 * BOARD_INSET, node.depth),
+            materials.frame,
+          ),
           'frame',
         )
         divider.name = `shelf-divider-${r}-${c}`