Review follow-up on the #76142 salvage: MemoryStore path-resolves and
shares one process-wide connection per file, so MemoryStore(":memory:")
creates a literal ./:memory: FILE whose state leaks across test runs —
the second run of the file failed all three spy tests because the
NULL-vector test had permanently wiped hrr_vector in the leaked db.
tmp_path isolates each run; verified two consecutive runs green + full
tests/plugins/memory/ green.
