subtreeCacheGateRejectCount

Number of cached subtrees rejected by the splice gate (directChildSizesMatch). Counts only true rejections — nodes that were recorded as subtree roots in the previous pass but whose direct-child sizes changed in this pass (typically a flex-grow cascade). Nodes that were never recorded as subtree roots (e.g. below MIN_SUBTREE_SCOPES) are NOT counted here, since they were never eligible to splice in the first place.