Fix space regen doubling Z location

Removing translate_obj_to_z_location from the existing-IfcSpace
regeneration branch. The ShapeBuilder rewrite (d8de62308) builds
geometry in local space preserving obj.matrix_world, making the
translate call redundant — it adds z on top of the already-correct
location.z, producing 2*z.

Add test_regenerate_space_preserves_z_location to cover the
regeneration path with a non-zero Z elevation.

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