Function buildStepExclusions

Pair-exclusions from IFC relationships. Only relationship getters (voids/filledBy/decomposedBy/decomposes) are used here; these read the relationship graph and never call extractEntityAttributesOnDemand, so the per-element loop stays off the AGENTS.md hot-loop anti-pattern:

• host vs the filler of its opening (wall vs door/window)
• element vs its own (meshed) opening
• members of the same IfcRelAggregates assembly