Function evaluate_synth_gates 

fn evaluate_synth_gates(
    db: &Arc<Db>,
    manifests: &SignedManifestSet,
    quarantines: &QuarantineSet,
    record: &HostRolloutState,
) -> Option<String>

Run the dispatch-gate evaluation against a Pending HRR record so maybe_synthesize_recovery_completion can refuse to fast-forward Pending → Converged when an active gate (channel-edges, wave-promotion, quarantine, etc.) would still hold the host.

Returns Some(reason) when a gate blocks; the caller leaves the host in Pending and lets the next regular plan tick handle it.