merge

This sketch folded with other, keeping at most maxPaths paths.

Exactly associative and commutative while the union fits the budget — the case the property tests assert, and the case that holds for any store whose documents have a bounded shape. Beyond the budget it is not, and cannot be: which paths survive depends on which observations have been seen when the truncation happens, and that is the fold order. What remains true unconditionally is conservation: observationCount and the sum of tracked and dropped observations are preserved whatever the order, so the totals a report is built on never depend on it.

The alternative — truncating only at the very end — would be exactly associative and would hold every path of every segment in memory until then, which for a store with a hundred segments is a hundred times the budget. Bounded memory is worth the caveat, and the caveat is written down rather than discovered.