Collection-per-file mode: concurrent explicit transactions corrupt the free-space index (IndexOutOfRangeException in SnapshotForTransaction) #325

Summary

In collection-per-file mode (CollectionDataDirectory), concurrent explicit transactions on one collection throw IndexOutOfRangeException from FreeSpaceIndex.SnapshotForTransaction (src/BLite.Core/Storage/FreeSpaceIndex.cs ~191). The free-space index's Dictionary is mutated concurrently and left inconsistent.

Cause

In that mode the free-space index is used without its _gate, and its OnCommit/OnRollback handlers run outside the collection lock, so two transactions read and write the same Dictionary at once.

Repro

8 writers × 50 explicit transactions (begin, insert, commit) into one collection of an engine opened with CollectionDataDirectory: 20 of 400 transactions throw IndexOutOfRangeException on main 3648a02. Single-file mode is not affected. Found during the review of #310, which does not cause it (6–37 of 400 with that PR applied).

Expected

No exception; the free-space index is updated under the same synchronisation as in single-file mode.