Grounded design answering the five open questions in #8075 with a
measured coupling census of release/v3.8.51: 126 db modules on the sync
prepare()/transaction() shape, 36 pragma sites, 159 SQLite migrations,
9 sqlite-vec + 8 FTS5 files, and a 4-driver SqliteAdapter seam that is
SQLite-only in practice (synchronous API).
Proposal: dialect-agnostic async DurableStateStore port with per-domain
repositories adopted incrementally; Postgres-first with MySQL gated by
a shared dialect-conformance suite; DATABASE_DRIVER config surface with
sqlite as the non-negotiable zero-config default; fail-closed one-way
export tool; five-phase delivery plan.
