Fixes a real-world failure where ifczip files distributed with a plain
`.ifc` extension (ACC / Dalux / Trimble Connect convention) errored as
"Not an ISO-10303-21 STEP file" because the Python header parser ran
before the Rust ZIP-aware source::open. Sannergata_RIV.ifc (128MB ZIP,
IFC2X3) parses cleanly now: 143k products, 0 empty, 28s end-to-end.