Phase 4 follow-on: registry-driven floor-plan rendering + shelf port

Adds the floor-plan side of the three-checkbox composition model
documented in wiki/architecture/node-definitions.md. Mirrors the 3D
side (def.geometry → <GeometrySystem> → <ParametricNodeRenderer>) but
emits SVG primitives instead of three.js Object3Ds, and runs inside
the floor-plan panel.

Type-side (packages/core/src/registry/types.ts):
 - New FloorplanGeometry tagged union covering path / polygon /
   polyline / rect / circle / line / group. FloorplanStyle props map
   straight to SVG attributes. Coordinates are level-local meters;
   rotations are radians (three.js convention).
 - New `def.floorplan?: (node, ctx) => FloorplanGeometry | null` field
   on NodeDefinition, independent of `geometry` and `renderer`. Re-
   exported via packages/core/src/registry/index.ts.

Runtime (packages/editor):
 - <FloorplanGeometryRenderer> walks the FloorplanGeometry tree and
   emits the matching React-SVG elements. Pure data → DOM; no per-kind
   logic.
 - <FloorplanRegistryLayer> reads the active levelId, walks the level
   subtree, looks up each node's def.floorplan, builds a
   GeometryContext, calls the builder, and renders the output via
   FloorplanGeometryRenderer.
 - Mounted in floorplan-panel.tsx just before <FloorplanMarqueeLayer>
   so registry-driven kinds layer above legacy inline content.

Shelf migration (proof port):
 - New nodes/src/shelf/floorplan.ts — buildShelfFloorplan(node) emits
   a group with the rotation/translation transform and a width × depth
   rectangle in the shelf's color. Brackets omitted (hidden under top
   board from above).
 - Wired to shelfDefinition.floorplan. Shelf now appears in the floor
   plan view for the first time (was missing from the legacy panel's
   inline switch).

Pattern proven; every future kind migrating in Phase 5 follows the
same shape: a pure (node, ctx) => FloorplanGeometry function. As kinds
register their floor-plan builders, the corresponding inline branches
in floorplan-panel.tsx become dead code and can be deleted in the
same PR.

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

packages/nodes/src/shelf/floorplan.ts

@@ -0,0 +1,43 @@
+import type { FloorplanGeometry } from '@pascal-app/core'
+import type { ShelfNode } from './schema'
+
+/**
+ * 2D floor-plan representation of a shelf. The top board (the largest
+ * visible surface from above) projects to a rectangle of `width × depth`
+ * centered on `(position.x, position.z)`, rotated by the shelf's Y angle.
+ *
+ * Brackets are intentionally omitted — they're hidden under the top
+ * board from a top-down view, and adding them as separate rects clutters
+ * the plan without conveying useful information at typical zoom levels.
+ *
+ * Coordinates are level-local meters; the floor-plan panel applies the
+ * world→SVG transform via its viewBox. Rotation is radians (three.js
+ * convention); the renderer converts to SVG degrees.
+ *
+ * Pairs with `buildShelfGeometry(node)` — the 3D builder. Same shape,
+ * different output projection.
+ */
+export function buildShelfFloorplan(node: ShelfNode): FloorplanGeometry {
+  const [px, , pz] = node.position
+  const ry = node.rotation[1] ?? 0
+  const halfW = node.width / 2
+  const halfD = node.depth / 2
+
+  return {
+    kind: 'group',
+    transform: { translate: [px, pz], rotate: ry },
+    children: [
+      {
+        kind: 'rect',
+        x: -halfW,
+        y: -halfD,
+        width: node.width,
+        height: node.depth,
+        fill: node.color,
+        stroke: '#1f2937',
+        strokeWidth: 0.015,
+        opacity: 0.9,
+      },
+    ],
+  }
+}