backfill

Whether Rabosh.open builds derived data for segments that have none before it returns.

true is what "model later" means for a store that has been running without either layer: one pass over the segments, on the calling thread, and the model is complete when open returns. The cost is proportional to the store, so a large one opens slowly the first time and quickly ever after — a segment already covered is skipped without being read.

false attaches to whatever sidecars exist and returns immediately. Nothing is wrong with the result: uncovered segments are scanned by queries, coverage reports the state honestly, and Rabosh.attach finishes the job whenever it suits. What it is not is a mode where anything silently answers from partial data — that distinction is the layers' own, and the facade only chooses when to pay.