Adjustments pass: shadow bias, polygon editor UX, space-detection load fix (#502)

* fix(viewer): tune shadow biases to stop acne without detaching shadows

normalBias 0.02 was too small a texel offset for the building-fit 1024
shadow map and brought back self-shadowing acne. Settle on normalBias 0.08
(0.07 and below acnes, 0.1 reads detached) plus depth bias -0.0005 to
suppress the residual acne that a normal bias alone couldn't clear.

Also adds a `?debug=shadowcamera` diagnostic that draws a CameraHelper for
each shadow camera so the fitted frustum can be inspected while tuning.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* feat(editor): polygon editor handle UX — focus, cursors, visibility

- vertex/midpoint cylinders ignore scene depth like the edge arrows so
  they stay visible through walls and slabs; vertex radius 0.1 -> 0.08,
  midpoint 0.06 -> 0.05, midpoints use the brighter arrow shade at rest
- during any drag only the active handle stays mounted (other arrows,
  vertices, edge bars and the cross disappear) so the gesture reads clearly
- handles set pointer cursors: move for vertices/midpoints/cross, and a
  screen-space direction-aware resize cursor for the edge arrows

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* fix(editor): keep ceiling affordances quiet during any interaction

The ceiling corner brackets' hit boxes caught drag-time hover (spatial
events keep firing during host drags by design), set hoveredId to the
ceiling and flashed the ceiling grid mid-gesture — e.g. while dragging a
slab polygon vertex. Unmount the brackets while ANY interaction scope is
active, and gate CeilingSystem's hover-driven grid reveal on idle scope +
no inputDragging as a second layer.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* fix(core): stop room detection resurrecting deleted slabs on load

Two holes in the wall-driven auto slab sync:

- initSpaceDetectionSync started with an empty baseline, so scene
  hydration (one atomic setScene) read as 'every wall changed' and ran a
  full detection pass on every load, recreating auto slabs the user had
  deleted. Seed the baseline from the store at init and treat a level's
  first snapshot as baseline — detection now only reacts to in-session
  wall edits.

- matchesManualFootprint required mutual coverage, so a single manual
  slab spanning multiple rooms never suppressed those rooms' auto slabs
  (only a fraction of it lies inside each room). Suppression now only
  asks whether the ROOM is substantially covered by the union of manual
  slabs.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

---------

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

packages/core/src/lib/space-detection.ts

@@ -252,15 +252,14 @@ function polygonCoverageRatio(subject: Point2D[], covers: Point2D[][]) {
 }
 
 // Demoted auto surfaces keep their polygon untouched, so a re-closed room
-// usually hits the exact-signature manual check. Coverage also handles a room
-// deliberately split across multiple manual surfaces: their union suppresses
-// a replacement auto surface as long as the pieces substantially belong to
-// and cover the room.
+// usually hits the exact-signature manual check. Coverage handles the rest:
+// a room split across multiple manual surfaces AND a single manual surface
+// spanning multiple rooms both suppress a replacement auto surface — what
+// matters is that the ROOM is already substantially covered, not that any
+// one manual surface belongs to it (a per-surface "mostly inside the room"
+// filter dropped multi-room slabs and resurrected deleted auto slabs).
 function matchesManualFootprint(roomPolygon: Point2D[], manualPolygons: Point2D[][]) {
-  const roomManualPolygons = manualPolygons.filter(
-    (manual) => polygonCoverageRatio(manual, [roomPolygon]) >= ORPHAN_MERGE_COVERAGE_THRESHOLD,
-  )
-  return polygonCoverageRatio(roomPolygon, roomManualPolygons) >= ORPHAN_MERGE_COVERAGE_THRESHOLD
+  return polygonCoverageRatio(roomPolygon, manualPolygons) >= ORPHAN_MERGE_COVERAGE_THRESHOLD
 }
 
 function pointDistanceToPolygonBoundary(point: Point2D, polygon: Point2D[]) {
@@ -1357,7 +1356,11 @@ export function isSpaceDetectionPaused(): boolean {
 }
 
 export function initSpaceDetectionSync(sceneStore: any, editorStore: any): () => void {
-  const previousSnapshots = new Map<string, string>()
+  // Baseline from whatever is already in the store. Detection reacts to wall
+  // edits made IN-SESSION (create / move / delete); it must not re-litigate a
+  // scene that merely loaded — rerunning on hydration resurrected auto slabs
+  // the user had deleted in an earlier session.
+  const previousSnapshots = levelStructureSnapshots(sceneStore.getState().nodes)
   let isProcessing = false
 
   const unsubscribe = sceneStore.subscribe((state: any) => {
@@ -1380,7 +1383,13 @@ export function initSpaceDetectionSync(sceneStore: any, editorStore: any): () =>
 
     const levelsToUpdate = new Set<string>()
     for (const levelId of new Set([...previousSnapshots.keys(), ...currentSnapshots.keys()])) {
-      if ((previousSnapshots.get(levelId) ?? '') !== (currentSnapshots.get(levelId) ?? '')) {
+      // First sight of a level is a hydration baseline, not a wall edit —
+      // `setScene` delivers a loaded scene as one atomic update, and a level's
+      // first wall can't close a room anyway. Record it (below) and only
+      // react to subsequent changes.
+      const previous = previousSnapshots.get(levelId)
+      if (previous === undefined) continue
+      if (previous !== (currentSnapshots.get(levelId) ?? '')) {
         levelsToUpdate.add(levelId)
       }
     }