One prepare call per turn, before input promotion: it creates the
checkpoint when missing, rebaselines after completed compaction, and
otherwise narrates drift as a chronological update. The initialize/
prepare two-step and its redundant queries are gone, and context update
messages now precede the promoted user message.
An undecodable stored applied record heals by treating every source as
new instead of failing the drain; ContextSnapshotDecodeError is deleted.
Renames: SessionContextEpoch -> SessionContextCheckpoint. The SQL table
name and wire event type are unchanged.
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