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test(worddiff): pin each cost guard in isolation, correct the budget's godoc

The byte pre-filter subtest used two identical prose lines, which the cell budget and the
identical-lines branch both reject on their own — it passed with the pre-filter deleted. It now
uses a single-token pair that clears every other gate, with a control pair sized to fit the
pre-filter, so the assertion can only be answered by the byte gate.

The maxDiffCells godoc said the budget was paid once per file load. It bounds one pair;
recomputeIntraRanges calls ComputeIntraRanges once per paired remove/add line, so a diff holding
many long pairs pays it for each.

Related to #323