editor: level-scoped alignment, reference-floor symbols & registry slab tool

[sudhir9297]

What does this PR do?

Follow-up to #372 (merged). Tightens the floor-plan / 3D alignment system and brings a few registry-driven kinds to parity.

• Level-scoped alignment candidates. Alignment now drops nodes that resolve to a different level, so a node directly below on another floor no longer snaps (alignment is XZ-only). Building-/site-scoped nodes with no level ancestor (e.g. an elevator shaft, which spans every floor) stay in the pool. The 2D floor plan deliberately keeps cross-floor alignment so you can still align to the reference floor below.
• Elevator footprint = outer shaft, not cab. The alignment footprint is derived from the outer shaft (shaft + wall, what's actually drawn) instead of the inset cab. The cab corners sat ~9 cm inside the visible edge — past the 8 cm snap — so the elevator never surfaced a guide. Default elevator width/depth bumped to 1.84 m to match.
• Shared stair footprint geometry. Extracted rotateXZ, computeSegmentTransforms, and stairFootprintAABB into stair-footprint.ts so slab-opening sync and alignment anchors derive the chain identically. Stairs now contribute plan-bbox anchors (straight = segment chain, curved/spiral = annular sector).
• Reference-floor registry symbols. Stairs/roofs/elevators/shelves/spawns on the reference floor now render through their registry def.floorplan builders, so the plan symbol is pixel-identical to one on the active floor.
• Registry-driven slab creation. Slab creation moved to the registry slab tool (parity with ceiling): the 2D handlers only maintain draft state and forward the closing double-click; the 3D tool commits the node, so a slab is no longer built twice.
• 3D guide ribbon tracks the active level. The alignment-guide ribbon is lifted to the active level's building-local Y each frame, so it lies on the floor being edited when floors are stacked.

How to test

1. bun dev, open a scene, switch to the 2D floor plan.
2. Stack two floors. Place a stair/elevator on floor 1, then move a node on floor 0 directly above/below it → it should not snap across floors (3D), while the 2D reference floor still aligns to the floor below.
3. Place an elevator → it now surfaces an alignment guide against nearby geometry (the guide derives from its outer shaft outline).
4. Place/move a stair (straight, curved, and spiral) → it shows alignment guides off its plan footprint.
5. On the reference floor, confirm stairs/roofs/elevators/shelves/spawns render the same plan symbol as on the active floor.
6. Draw a slab in the floor plan → exactly one slab is created (no duplicate).
7. With floors stacked, drag a node and confirm the 3D guide ribbon sits on the floor being edited, not the building base.
8. bun check and bun check-types should pass.

Screenshots / screen recording

N/A in this description — change is visual/interactive; a screen recording of the cross-floor alignment + elevator/stair guides will be added on the PR. Reviewers can reproduce via the steps above.

Checklist

• I've tested this locally with bun dev
• My code follows the existing code style (run bun check to verify)
• I've updated relevant documentation (if applicable)
• This PR targets the main branch