is At Or Below
True when prefix is at the same segment boundary as a prefix of candidate — i.e. either equals it or names an ancestor in the dotted-bracket tree.
Examples:
isAtOrBelow("scenarios[3]", "scenarios[3]")→ true.isAtOrBelow("scenarios[3]", "scenarios[3].runOverrides.x")→ true.isAtOrBelow("scenarios[3]", "scenarios[30].x")→ false.isAtOrBelow("scenarios", "scenarios[3].x")→ true (whole subtree).isAtOrBelow("", anything)→ true (empty prefix matches all).