Why: Project user messages and final assistant replies need exact, replayable source parts before a domain observer can safely analyze them.
Evidence: go test -p 1 ./... passed; isolated PostgreSQL outbox, audit, migration, and work-context tests passed against a temporary cluster; request dedup, fenced settlement rollback, authorization recheck, and receipt idempotency tests passed.
Impact: Add migration 44 and a private host outbox for committed message parts; actual observer delivery and action preflight are not yet wired into the product path.
Refs: TT-N03, TT-R01; approved X3 stage-five plan r1
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