Builder 2D: tag objects with factory/floor (enable 2D kawasan authoring)

[digtaalfathir]

Prerequisite for the 2D kawasan view. The 2D layout export (buildLayoutJSON) currently emits {viewBox, rooms, walls, pins} with no factory tags.

Mirror the 3D Builder: tag each object with factory (+ optional floor) and emit a factories manifest, so the 2D kawasan can be authored instead of merged by hand.