fix(walls): bound wall miters, heal corrupt scenes, follow grid snap

Three related editor fixes surfaced while debugging a captured house
project that rendered an infinite wall and failed to load.

Infinite wall (core/systems/wall/wall-mitering.ts):
  Junction miters are line-line intersections, so the joint point sits
  ~halfThickness/sin(theta) from the junction. The only guard was an
  exact-parallel check (det < 1e-9), so two walls meeting at a shallow
  angle (a room-preset preview dragged onto an existing wall, or a wall
  drawn nearly collinear to its neighbour) produced a joint point far
  away — an infinite spike. Add a miter limit: reject joints farther
  than 10x half-thickness from the junction and fall back to a square
  joint, exactly like the parallel case.

Scene load failure (core/utils/heal-scene-graph.ts + validate-build-json
+ use-scene migrateNodes):
  Capture wall-merge could leave a `children: [null]` entry (see the
  matching merge-walls.ts fix in private-editor) and zero-length walls.
  `null` children fail wall schema validation, so the whole scene fails
  to load. Add a shared heal step — strip non-string child refs, drop
  childless zero-length walls — run on every load path: import
  validation now repairs instead of hard-failing (with a warning), and
  setScene heals on the prod project-load path too.

Grid snap (nodes slab/ceiling/spawn tools):
  These tools hardcoded a 0.5 m snap (Math.round(x*2)/2) and ignored the
  editor's grid-snap setting, so the cursor jumped by 0.5 while later
  vertices already followed the configured step. Route them through
  snapPointToGrid / snapScalar with gridSnapStep.

Adds unit tests for the miter limit and the heal step.

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

packages/nodes/src/slab/tool.tsx

@@ -6,6 +6,7 @@ import {
   type GridEvent,
   type LevelNode,
   snapPointAlongAngleRay,
+  snapPointToGrid,
   useScene,
 } from '@pascal-app/core'
 import {
@@ -76,9 +77,9 @@ export const SlabTool: React.FC = () => {
       if (!cursorRef.current) return
       const rawPoint: [number, number] = [event.localPosition[0], event.localPosition[2]]
       const bypassSnap = shiftPressed.current || event.nativeEvent?.shiftKey === true
-      const gridX = Math.round(rawPoint[0] * 2) / 2
-      const gridZ = Math.round(rawPoint[1] * 2) / 2
-      const gridPosition: [number, number] = bypassSnap ? rawPoint : [gridX, gridZ]
+      const gridPosition: [number, number] = bypassSnap
+        ? rawPoint
+        : [...snapPointToGrid(rawPoint, useEditor.getState().gridSnapStep)]
       setCursorPosition(gridPosition)
       setLevelY(event.localPosition[1])
       const lastPoint = points[points.length - 1]