Every event-derived ServerMessage gains sequence (global) and sessionSequence
(per-session) cursor fields. SessionSnapshot adds lastSequence and
lastSessionSequence at projection-read time. SnapshotComplete data object
terminates the snapshot phase and is registered in the polymorphic module.
replaySnapshot() now emits SnapshotComplete after all SessionSnapshot messages.
SessionStarted audit deferred with TODO comment. DomainEventMapper propagates
both cursors to all mapped messages including InferenceCompleted.
kami
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