Function ValidateCloseReasonCoherence

PURE invariant: CloseReason is meaningful only for a terminal bridge.

• a non-terminal status with a CloseReason set is contradictory (the bridge isn't closed).
• a terminal status SHOULD record a reason (returned as a warning, not a hard failure, so the janitor / error paths that haven't yet stamped a reason aren't blocked from persisting). Returns the error/warning, or null when coherent.