feat(editor): mode-driven shelf/column/spawn placement + cross-kind floor collision

Migrate the remaining floor-placed kinds onto the unified snapping/modifier
model and generalize floor collision so any solid floor kind blocks any other.

- shelf/column/spawn declare `snapProfile: 'item'` → contextual snapping chip,
  Shift=cycle, Ctrl=grid step during placement; their tools read the active
  mode (grid/lines/off) instead of legacy Shift/Alt bypass; spawn fresh
  placement now respects alignment ("lines") like its move.
- Resize/radial handles claim the handle-drag scope (new RESIZE_HANDLE_DRAG_LABEL)
  so the HUD shows no select-mode shortcuts mid-resize.
- Column move migrated to the generic MoveRegistryNodeTool (declare `movable`,
  drop the bespoke move-tool) — gains mode-driven snapping, alignment, R/T,
  slab lift, grid SFX, and the collision box for free. 2D move still routes
  through `floorplanMoveTarget`.
- Cross-kind floor collision: new declarative `FloorPlacedConfig.collides`
  (item/shelf/column opt in; spawn/MEP/stair stay off). `canPlaceOnFloor` now
  treats every colliding floor kind as an obstacle (was item-only), reading the
  declarative footprint; the generic move tool's red/green placement box gates
  on `collides`. Column footprint uses the visible `columnFootprintHalf` extent
  so the box/slab-lift/collision track the real (round/square) column size.

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

packages/nodes/src/item/definition.ts

@@ -225,6 +225,7 @@ export const itemDefinition: NodeDefinition<typeof ItemNode> = {
         return { dimensions: getScaledDimensions(item), rotation: item.rotation }
       },
       applies: (node) => !(node as ItemNodeType).asset.attachTo,
+      collides: true,
     },
     // Recessed ceiling fixtures cut a hole in their host ceiling. The viewer's
     // CeilingSystem queries this capability on each child of a ceiling so it