feat(editor): placement & interaction overhaul — FSM spine, bug tracks, perf

Implements plans/editor-placement-interaction-overhaul.md: an authoritative
interaction-scope state machine plus the catalogued placement/interaction
fixes, and split-view floor-plan performance.

- Interaction-scope spine (lib/interaction/* + store/use-interaction-scope),
  driven from central useEditor setters; overlay scoping (zone labels,
  context badges, floating action menu) reads resolveOverlayPolicy.
- Bug tracks A/B/D/E/F/G/H: handle/cutout raycast, footprint validity,
  auto-slab loop, ceiling hosting, B-key tool desync, 2D drop offset,
  per-frame jank.
- Snapping modes (grid/lines/angles/off) + contextual HUD chips; modifier
  model (Shift=cycle, Alt=free place, Ctrl=grid step).
- Item move now tracks the cursor 1:1 (was a laggy per-frame lerp); handle
  rig hides during a whole-node move; rotate gizmo advertises Shift=free
  rotation in the HUD and hides the move cross while rotating.
- Floor-plan perf: pause live reactivity while in 3D-only view; per-node
  geometry cache so only changed nodes rebuild on a drag; hoist wall miters
  to a once-per-pass ctx.levelData (O(N^2) -> O(N) on wall/opening drags).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

wiki/architecture/README.md

@@ -14,6 +14,7 @@ Canonical rules for code that touches `packages/core`, `packages/viewer`, `packa
 | [item-authoring](item-authoring.md) | Content-author contract for catalog item GLBs: `slot_` material naming, authored defaults + `pascal_material` extras, the `cutout` reserved mesh, UV world scale, and the validated Blender/export recipe |
 | [plugin-authoring](plugin-authoring.md) | Public contract for external plugins — `Plugin` shape, `setPluginDiscovery`, lifecycle, what's in and out of v1 |
 | [tools](tools.md) | Editor tools structure, 2D↔3D behavioral parity, manipulation constraints, and Shift bypass defaults |
+| [interaction-scope](interaction-scope.md) | The authoritative interaction state machine ("the spine"): `InteractionScope` union, the begin/update/end/endIf contract, the raycast hot-set, and the overlay scope matrix |
 | [viewer-isolation](viewer-isolation.md) | Keeping `@pascal-app/viewer` editor-agnostic |
 | [selection-managers](selection-managers.md) | Two-layer selection (viewer + editor), events, outliner |
 | [scene-registry](scene-registry.md) | Global node ID → Object3D map and `useRegistry` |
@@ -25,4 +26,5 @@ Canonical rules for code that touches `packages/core`, `packages/viewer`, `packa
 ## Reading order for an architecture review
 
 1. [layers](layers.md), [systems](systems.md), [renderers](renderers.md), [tools](tools.md), [viewer-isolation](viewer-isolation.md) — required every review.
+   - When the diff touches placement / move / handle / reshape / box-select / paint or any overlay or picking behaviour, also read [interaction-scope](interaction-scope.md).
 2. The remaining pages on demand, based on what the diff touches.