Phase 5 Stage C continued: wall, door, window now in registry floor plan

Three remaining kinds at Stage C this session:

wall → C
 - buildWallFloorplan: uses ctx.siblings to gather other walls in the
   level, runs calculateLevelMiters, computes plan footprint via
   getWallPlanFootprint. Same visual output as legacy.
 - getFloorplanWall thickness exaggeration inlined (~25 lines from
   editor/lib/floorplan/walls.ts) to keep nodes/wall self-contained.
 - floorplan-panel.tsx's wallPolygons short-circuits to [] when wall
   is registered.
 - Performance note: recomputes level miter data per wall (O(N²) for N
   walls in a level). Acceptable for typical scenes; ctx.levelData?.
   miters optimization deferred to Stage B's wall design pass.

door → C
 - buildDoorFloorplan: inlines getOpeningFootprint math from
   floorplan-panel.tsx (40 lines, pure math). Uses ctx.parent as the
   wall to compute direction + perpendicular for the cutout footprint.
 - Returns null when parent isn't a wall (orphaned doors during
   placement).

window → C
 - buildWindowFloorplan: same shape as door, glass-blue tint to
   distinguish visually.

Both share the legacy openingsPolygons gating:
 - floorplan-panel.tsx's openingsPolygons useMemo filters per kind so
   a partial migration still works (e.g., if only door registers, only
   doors get skipped). When both registered, returns [] entirely.

Item C intentionally deferred — needs parent-chain transform helpers
(buildFloorplanItemEntry / getItemFloorplanTransform from editor/lib/
floorplan/items.ts) exposed publicly or moved into core. A focused
session is the right place to design that boundary.

Stage B for door / window / wall still pending — each is a focused
session per kind (large geometry math extractions, wall needs ctx.
levelData design).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/nodes/src/door/floorplan.ts

@@ -0,0 +1,60 @@
+import type {
+  DoorNode,
+  FloorplanGeometry,
+  FloorplanPoint,
+  GeometryContext,
+  WallNode,
+} from '@pascal-app/core'
+
+/**
+ * Stage C floor-plan builder for door. Doors render as a small polygon
+ * sitting in the wall's cutout — width = door.width along the wall
+ * direction, depth = wall.thickness perpendicular.
+ *
+ * Requires `ctx.parent` to be a wall (door.parentId is the wall it's
+ * mounted on). Returns null when the parent isn't a wall (orphaned
+ * doors during placement etc.).
+ *
+ * Inlined from the legacy `getOpeningFootprint` helper in
+ * floorplan-panel.tsx. Window's builder is structurally identical.
+ */
+export function buildDoorFloorplan(node: DoorNode, ctx: GeometryContext): FloorplanGeometry | null {
+  const wall = ctx.parent as WallNode | null
+  if (!wall || wall.type !== 'wall') return null
+
+  const [x1, z1] = wall.start
+  const [x2, z2] = wall.end
+  const dx = x2 - x1
+  const dz = z2 - z1
+  const length = Math.sqrt(dx * dx + dz * dz)
+  if (length < 1e-9) return null
+
+  const dirX = dx / length
+  const dirZ = dz / length
+  const perpX = -dirZ
+  const perpZ = dirX
+
+  const distance = node.position[0] // door's local X = distance along wall
+  const width = node.width
+  const depth = wall.thickness ?? 0.1
+  const cx = x1 + dirX * distance
+  const cz = z1 + dirZ * distance
+  const halfWidth = width / 2
+  const halfDepth = depth / 2
+
+  const points: readonly FloorplanPoint[] = [
+    [cx - dirX * halfWidth + perpX * halfDepth, cz - dirZ * halfWidth + perpZ * halfDepth],
+    [cx + dirX * halfWidth + perpX * halfDepth, cz + dirZ * halfWidth + perpZ * halfDepth],
+    [cx + dirX * halfWidth - perpX * halfDepth, cz + dirZ * halfWidth - perpZ * halfDepth],
+    [cx - dirX * halfWidth - perpX * halfDepth, cz - dirZ * halfWidth - perpZ * halfDepth],
+  ]
+
+  return {
+    kind: 'polygon',
+    points,
+    fill: '#f8fafc',
+    stroke: '#374151',
+    strokeWidth: 0.015,
+    opacity: 0.95,
+  }
+}