The cache-only Steer branch (no AGENT_INSTANCES worker) only rejected
*conflicting* ownership, so a missing stream owner or a missing
active-run session_id was treated as non-conflicting and let
agent.steer() run and report accepted without proving the requesting
session owns the run.
Fail closed instead: both the stream owner and the active-run session
must be present and strictly equal to the requesting session (backend
and phase are still revalidated) under the same STREAMS_LOCK ->
ACTIVE_RUNS_LOCK edge; any absent identity returns stream_dead without
calling agent.steer(). No cache eviction, no agent closing, HTTP write
still outside the locks.
Regressions: missing owner, missing/empty run session, both missing,
missing run entry -> stream_dead + steer never called; both present and
equal -> accepted, cache untouched. Legacy happy-path fixtures now
register the ownership they rely on.
Anthony Ruiz
20c16e9167