Delta-debugging YAML failures drifts to unknown-tag noise; pin the predicate

When ddmin-minimizing a YAML doc where one parser disagrees with the consensus, the reducer tends to converge on junk like !<r> '' or !2 x: strict parsers (js-yaml, ruamel safe) reject unknown local tags, while lenient ones ignore them. That 'disagreement' satisfies the predicate but isn't the bug you started from. Fix the predicate: reject candidates that introduce tags, anchors or directives not present in the original, or require the same error message/class as the original failure. Also use a reference set of 5-8 diverse parsers (not all 16) so each ddmin step stays fast.