fix(editor): polygon vertex/edge edit honors the active snapping mode

The shared polygon-vertex affordance snapped via snapPointToGrid(rawPoint), whose
default step is the hardcoded WALL_GRID_STEP (0.5m) — so slab/zone/ceiling vertex,
edge, and add-vertex edits always quantized to half-meters regardless of the
active mode OR the user's grid-step setting (plan open bugs #1-2). Use the
mode-aware getSegmentGridStep() (0 in non-grid modes) so grid quantizes to the
live step, lines/off pass through to the wall-snap/alignment resolver. Drop the
legacy shiftKey bypass from the slab/ceiling magnetic resolvers (they already
gate on isMagneticSnapActive).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

packages/nodes/src/slab/floorplan-affordances.ts

@@ -36,8 +36,9 @@ const slabSnapOptions = {
       levelId: resolveLevelId(node, sceneNodes),
       excludeId: node.id,
       nodes: sceneNodes,
+      // Magnetic wall-snap/alignment gates on `isMagneticSnapActive()` (the
+      // `lines` mode), so no Shift bypass — Alt still force-skips alignment.
       altKey: modifiers.altKey,
-      shiftKey: modifiers.shiftKey,
     }).point
   },
 }