floorplan/zone: name label inside polygon + polygon editor on select

`buildZoneFloorplan` now emits a centered name label at the polygon's
area-weighted centroid (Shoelace formula, with bbox-center fallback for
degenerate rings). Label uses the legacy `FloorplanZoneLabel` styling:
`fontSize: 0.2`, white fill, zone-color stroke, `paintOrder: 'stroke'`
for the "outlined text" look that stays legible above any fill.

When the zone is selected the builder also emits the polygon editor —
edge-handle per edge, midpoint-handle per midpoint, endpoint-handle per
vertex — driven by the shared `createPolygonVertexAffordance` /
`createPolygonAddVertexAffordance` / `createPolygonMoveEdgeAffordance`
factories slabs and ceilings already use. Zones have no `holes` field
so the factory's optional `holeIndex` stays undefined and the operations
target `node.polygon` directly.

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

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

@@ -0,0 +1,20 @@
+import type { ZoneNode } from '@pascal-app/core'
+import {
+  createPolygonAddVertexAffordance,
+  createPolygonMoveEdgeAffordance,
+  createPolygonVertexAffordance,
+} from '../shared/polygon-vertex-affordance'
+
+/**
+ * 2D drag affordances for zone — same three polygon-editing operations
+ * slabs and ceilings expose. Zones have no `holes` field, but the
+ * shared factory accepts that case (holeIndex stays undefined and the
+ * boundary polygon is the target).
+ *
+ *   - `move-vertex` — drag an existing polygon vertex.
+ *   - `add-vertex` — insert a new vertex at an edge midpoint, then drag.
+ *   - `move-edge` — drag an entire edge perpendicular to itself.
+ */
+export const zoneMoveVertexAffordance = createPolygonVertexAffordance<ZoneNode>('zone')
+export const zoneAddVertexAffordance = createPolygonAddVertexAffordance<ZoneNode>('zone')
+export const zoneMoveEdgeAffordance = createPolygonMoveEdgeAffordance<ZoneNode>('zone')