The columnar index the model was built from. Hand to
serializeIndexSidecarFromColumns (with a source hash) to produce the
revisit sidecar — the columns ARE the sidecar payload (M7 identity).
The STEP header (available even on some failed parses).
The model over the windowed source (undefined if the parse failed).
The parse result.
Window diagnostics from the streaming build.
The result of a streamed IFC open: the windowed-source model plus the artifacts a consumer needs around it.