fix: zone grid snapping, shelf position panel, deselect on item placement (#394)

* fix: zone grid snapping, shelf position panel, deselect on item placement

- Zone tool reads the editor's gridSnapStep (0.5/0.25/0.1/0.05) instead
  of a hardcoded 0.5 for both cursor move and click snapping.
- Shelf inspector gains a Position group (vec3 X/Y/Z sliders), matching
  the item panel.
- Item catalog clears the viewer selection before arming placement so
  shortcuts (rotate & co) don't hit both the ghost and the selected node.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* refactor: move level display naming into @pascal-app/core

getDefaultLevelName / getLevelDisplayName ("Ground Floor" / "Floor N" /
"Basement N") lived in packages/editor's internal lib, so viewer-only
surfaces couldn't reach them and fell back to hand-rolled "Level N"
labels. The helpers are pure domain logic, so they move to core and
export from its barrel; the editor package's seven call sites now import
them from there.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

---------

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

packages/nodes/src/shelf/parametrics.ts

@@ -76,5 +76,9 @@ export const shelfParametrics: ParametricDescriptor<ShelfNode> = {
         { key: 'height', kind: 'number', unit: 'm', min: 0.05, max: 2.5, step: 0.05 },
       ],
     },
+    {
+      label: 'Position',
+      fields: [{ key: 'position', kind: 'vec3' }],
+    },
   ],
 }