Unclassifiable occurrences. These force unknown only when no proven write
exists; rule 1 may therefore return true with blockers still reported.
Located structural surprises from the segmenter / bounded traversal, plus
the bom/non-ascii positional facts. Any NON-POSITIONAL defect abstains
(see hasStructuralDefect); the positional two never do.
How many occurrences were proven writes. Agreed with IL 100% on both splits.
Q16 result: the tristate plus everything the caller needs to see why.