Always enforce session execution lease invariants with regression tests

When changing session execution, recovery, heartbeat, interruption, or cancellation, add deterministic production-path regression tests for strict ownership fencing. Prove that live leases never admit a second owner or recovery, uncertain ownership cannot re-enter, expired claims are not overwritten or revived by heartbeats, newer assistants exclude older recovery, stale owners cannot overwrite takeover state, and runner settlement cannot overwrite cancellation. Coordinate races explicitly with Effect primitives, gates, or database triggers, and assert durable claim/recovery state plus the absence of duplicate execution, continuations, wakes, stale projections, and other forbidden side effects.