Two invariants layered on the origin-routing commit (#55578):
1. Fail closed on orphaned async-delegation payloads. The poller's
belongs-elsewhere check handles events owned by another LIVE session,
but an event whose owner is gone previously fell through and was
adopted by whichever poller saw it - injecting one chat's delegation
output into another chat. Delegation completions are now injected
only into a session that PROVABLY owns them (origin UI id, or
session-key/lineage match via the compression chain); unowned
payloads are dropped from injection with a WARNING (the subagent's
output is already persisted in the delegation records, so nothing is
lost). The shutdown drain applies the same rule. Non-delegation
events keep the historical adopt-orphans behavior.
2. A session's in-flight async delegations end with the session.
_finalize_session now calls interrupt_for_session(): delegations
commissioned by the closing UI session are interrupted always;
key-matched delegations only when the TUI owns the session lifecycle,
so closing a viewer tab on a live gateway session never kills the
gateway's own background work.
