floorplan/registry-layer: z-order buckets + overlay pass + click guard

Three layered improvements on the 2D registry layer:

- Z-order buckets so the SVG document order reflects intent: zones (0)
  paint first, slabs/ceilings (1) next, every other kind (walls / items /
  shelves / columns / stairs / …) on top. Stable sort preserves DFS order
  within a bucket.

- Base / overlay split: each entry's `FloorplanGeometry` tree is walked
  through `splitFloorplanOverlay`, partitioning into a base group
  (polygons, paths, fills, hatches) and an overlay group (interactive
  handles + labels — `text` / `endpoint-handle` / `midpoint-handle` /
  `edge-handle` / `move-handle` / `dimension` / `dimension-label`). Base
  renders rank-sorted; overlays paint after every base entry so polygon-
  editor chrome on a selected slab and zone name labels stay legible
  above the structural fills sitting on top of them.

- Click guard on the outer layer `<g>`. The base/overlay split means
  pointer-down lands on base and pointer-up lands on overlay (selection
  mounts the overlay on top mid-gesture). The browser then dispatches
  `click` to the lowest common ancestor, ABOVE the entry's
  `onClick={handleClickStop}`. Without a higher-level stop, the click
  reached the SVG's `handleBackgroundClick` → `clear-elements` and the
  selection set on pointer-down vanished a frame later. Scoped to
  `onClick` only so pointer / hover / drag still propagate inside the
  registry tree.

Also extends the `text` FloorplanGeometry with stroke / strokeWidth /
paintOrder / fontFamily so kinds can match the legacy "white fill +
colored outline" label look (used by the new zone name label).

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

packages/core/src/index.ts

@@ -9,11 +9,13 @@ export type {
   EventSuffix,
   FenceEvent,
   GridEvent,
+  GuideEvent,
   ItemEvent,
   LevelEvent,
   NodeEvent,
   RoofEvent,
   RoofSegmentEvent,
+  ScanEvent,
   ShelfEvent,
   SiteEvent,
   SlabEvent,