Resolve this backend's independent local artifact for a session without reading, creating, flushing, or otherwise materializing it. Backends such as SQLite that do not own one artifact per session return `undefined`.
- `meta` — the immutable session header whose artifact is requested.
**Returns** the backend-specific absolute location, when one exists.
Register a new session's metadata. A backend MAY defer the physical write until the first append (lazy materialization), in which case a created-but-never-appended session is absent from list — abandoned sessions leave nothing behind.
- `meta` — the immutable header (id, version, cwd, lineage) to record.
Durably persist a batch of events (called from the write-behind drain at the `session/flush` checkpoint). Honors the append-only and contiguous-seq contracts: the first event's `seq` MUST equal the stored next-seq (after `load` has durably closed any interrupted turn). Rejects non-JSON- serializable `event.data` with an error naming the offending event type.
- `id` — the session the batch belongs to.
- `events` — the contiguous batch to persist, in seq order.
Load a header and balanced contiguous log. A complete interrupted final turn is preserved and durably closed with missing tool errors plus any open step and turn boundaries; only a torn final record is discarded. Unknown versions and corruption in the committed prefix reject.
