The staged-promotion machinery (concludingParents keyed by parent token
plus a pendingParentConclusions staging map) spread one fact — this call
concluded the turn — across three registry-side collections with manual
cleanup. Align it with how additionalContexts already crosses the same
boundary: concludeTurn() marks only its own execution, the marker rides
that execution's successful result (ToolExecutionFailure types
concludesTurn as never, so a policy-converted failure sheds it with the
type), and the composite that owns the nested dispatch forwards it —
Code Mode's binding does so beside its existing context forwarding.
The registry loses both parent-keyed collections and the promotion
block; the propagation decision moves to the owning boundary; the
structured-output consumer's own two-phase commit is untouched.