Two related bugs caused floor items to disappear mid-2D-drag:
1. `startLevelId` did `parent.parentId` unconditionally for non-wall
parents — so for a floor item parented directly to the level (the
canonical convention from `use-placement-coordinator`), it returned
`level.parentId = building.id`. `findContainingSurface` then iterated
the building's children (levels, not slabs) and the fallback
`parentId: startLevelId` reparented the item to the building. Now we
walk the parent chain until we hit a `level` and short-circuit on
`parent.type === 'level'`.
2. `buildSurfaceItemSession` reparented floor items to a slab when the
cursor was over one. Slabs don't carry a `children` field on their
schema (only ceilings + level do), so `updateNodesAction`'s reparent
logic operated on `undefined.children` — the item dropped out of the
level→children DFS the floor-plan layer walks, and the polygon
stopped rendering mid-drag. Split into `buildFloorItemSession`
(always parents to the level, just updates position) and a
ceiling-only `buildSurfaceItemSession`. `findContainingSurface`
narrows to `'ceiling'` as the only valid target.
Matches the 3D `detachItemSurfaceToFloor` convention: floor items live
as level children, not slab children.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
