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/core/src/validation/validate-build-json.ts

@@ -1,4 +1,5 @@
 import { AnyNode, type AnyNodeType } from '../schema/types'
+import { healSceneNodes } from '../utils/heal-scene-graph'
 
 export type ValidationSeverity = 'error' | 'warning'
 
@@ -127,9 +128,22 @@ export function validateBuildJson(input: unknown): ValidateBuildJsonResult {
     }
   }
 
-  const nodes = nodesRaw as Record<string, unknown>
+  // Heal known pre-existing corruption (null children, zero-length walls) up
+  // front, so a scene saved before the source fixes still imports instead of
+  // hard-failing schema validation. `parsed` below carries the repaired nodes.
+  const { nodes, droppedWallIds, strippedChildRefs } = healSceneNodes(
+    nodesRaw as Record<string, unknown>,
+  )
   const rootNodeIds = rootNodeIdsRaw as string[]
 
+  if (strippedChildRefs > 0 || droppedWallIds.length > 0) {
+    warnings.push({
+      severity: 'warning',
+      code: 'auto_repaired',
+      message: `Repaired on import: removed ${strippedChildRefs} invalid child reference${strippedChildRefs === 1 ? '' : 's'} and ${droppedWallIds.length} zero-length wall${droppedWallIds.length === 1 ? '' : 's'}.`,
+    })
+  }
+
   if (rootNodeIds.length === 0) {
     errors.push({
       severity: 'error',