OptionalonFired once, right after the STEP header parses — before the full file parse — with everything the header reveals, so callers can print the model line as early as possible (issue #301 follow-up, log line 3).
OptionalonStructured, throttled progress events across the load phases (headerParse / dataParse / geometry) — see core/progress.ts.
OptionalprogressMinimum ms between progress events (default in core/progress.ts).
OptionalyieldWhen true, the parse and geometry loops periodically await a macrotask so browsers can repaint between progress events and the tab is not flagged as stalled. Costs a few % wall clock; off by default.
Options threading the progress/yield contract (issue #301) through a load.