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The JSONL and SQLite backends were byte-identical (or same-algorithm) for ALL of their write-path orchestration — the four maps (states/buffers/chains/inits), installWritePath, initFor, onCreated's four adoption cases, flush, drain, serialize, adopt/adoptLivePrefix, assertVersion, and the create/append/load/ has/delete skeletons. Only the storage primitives (write bytes vs INSERT rows) differed, so every fix landed twice. Extract that orchestration into a PersistenceCoordinator in the seam package. Each backend composes one (new PersistenceCoordinator(ctx, this)), implements a small PersistenceBackend hook interface (loadStored, loadLive, appendBatch, commitRepair, deleteStored, list, optional close), and delegates its six public service methods to it. Composition, not inheritance — a backend exposes only the hooks, can't reach the coordinator's private state, and the public SessionPersistence API is unchanged so a third-party backend may still implement it directly. The crash-repair torn-tail token is OPAQUE: the coordinator computes the synthetic closers (it owns interruptedTurnClosers) but only tests `tornMarker !== undefined` and round-trips it to commitRepair, never inspecting it (JSONL = byte offset, SQLite = seq). loadStored vs loadLive stay distinct so HMR adoption is cwd-scoped (a same-id log at a different cwd is a collision, not a resume). appendBatch carries meta so lazy-materialize + first-batch commit atomically (no separate materialize hook). Tests: the duplicated orchestration tests (adoption, HMR, collision, dispose-drain, crash-tail) move into one runCoordinatorContract suite run once per backend (memory + jsonl + sqlite) via hook fixtures; per-backend specs keep only storage mechanics. A through-coordinator torn-tail test per real backend keeps the commitRepair-with-marker branch covered under the 100% gate. Net -112 lines (the dedup outweighs the new coordinator + shared suite); 100% coverage; backends shrank ~1200 lines of duplicated churn. Migrates the write-coordinator RFC proposed -> implemented.