fix: race condition where artifacts from late resolving flights could be silently dropped

[Bill Li (billxinli)]

• Fix race condition where fetchStrategy pushed into a shared artifacts array by reference late resolving flights could write into a stale array that had already been yielded, silently dropping results
• Change fetchStrategy to return artifacts instead of mutating a passed in array, so the scanner factory controls when results are merged via a closure that always reads the current array
• Add tests covering late-resolving flights