`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>
