The `materialized` INTEGER column was redundant: create()/update() already
keep a lazy session in memory and write no row, so a `sessions` row is
written only by the first append. Its EXISTENCE is the materialization
signal — has()/list() now report exactly the sessions that have a row,
matching the JSONL backend's "file exists ⇔ materialized".
The column only existed to force has()/list() to FALSE for an all-tail
crash (a partial first turn, zero committed events). That actually
DIVERGED from the JSONL backend, whose file (and thus has()=true) survives
a first append that never reached turn/end. Removing the column drops that
special case: an all-tail session keeps its row and stays present, the
same as JSONL. The orphaned tail rows are still removed by the deferred
truncation-repair on the next append, and load() stays non-mutating.
Also: add a TODO to route through a cordis db service if one is adopted,
and correct the README's Node-version framing to the repo's engines (>=24).
Tianyi Cui
3bef6b38c7