fix(shelf): match cubby divider depth to the boards (no front overflow, no back seam)

The cubby column divider was full-depth while the boards are depth-recessed,
so it (a) sat proud of the shelf fronts — the earlier Y-tuck then made it poke
past the top/bottom boards — and (b) shared the back panel's rear plane,
z-fighting down the centre of the back.

Give the divider the same depth recess as the boards (via boardGeometry) and
drop the Y-tuck: it now sits flush with the shelf fronts and its back tucks
inside the back panel. Flush top/bottom is fine — those board faces are
back-to-back, not co-facing, so they don't fight.

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

packages/nodes/src/shelf/geometry.ts

@@ -350,13 +350,12 @@ 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(
-          // Extend 1mm into the boards above + below (centre unchanged) so the
+          // Same depth recess as the boards: the divider sits flush with the
-          // divider tucks under the top board and onto the bottom board instead
+          // shelf fronts (not proud) and its back tucks inside the back panel,
-          // of meeting them on a coplanar face (which shimmers / reads merged).
+          // so it neither overflows the boards at the front nor z-fights the
-          new Mesh(
+          // back panel down the centre. Height is flush (the board faces it
-            new BoxGeometry(node.thickness, dividerHeight + 2 * BOARD_INSET, node.depth),
+          // meets top/bottom are back-to-back, so they don't fight).
-            materials.frame,
+          new Mesh(boardGeometry(node.thickness, dividerHeight, node.depth), materials.frame),
-          ),
           'frame',
         )
         divider.name = `shelf-divider-${r}-${c}`