Heartbeat Session Stream

A heartbeat's outcome: the reconciled stream plus the session's actual owner replica.

replica_id is the replica that currently holds the affinity key after the claim (this caller if it won or already held it, another replica otherwise). The runner reads it to refuse serving a local sandbox session it does not own.

stream is None when a losing replica heartbeats a session that has no row yet: it may not create or stamp one, since that row belongs to the owner.

is_current_turn (W7.4) is False when this turn_id's alive/running lock was gone or reassigned at the moment of this beat — i.e. a cancel/steer/kill interrupted this turn since the last heartbeat. The runner's watchdog reads this to abort the in-flight run; without it a cancel that raced a heartbeat's nx=True re-acquire would silently re-arm the SAME lock under the SAME turn_id and the interruption would never surface.