feat(benchmarks): expand multilingual stemming quality evaluation

* cover all Radixor dictionary languages
* add PRIMARY_OUTPUT, ANY_CANDIDATE, and ALL_CANDIDATES policies
* measure pairwise over-stemming and under-stemming
* add balanced accuracy and complementary quality metrics
* compare single-output and multi-output stemmers fairly
* improve result validation, reporting, and documentation
* move stemming quality tests into the standard test source set
* preserve the existing JMH benchmark structure and badge output

docs/benchmarks/data/stemming-quality.sha256

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+5a93a6ab60e46489737cd649eb1ac48182114b9038f7f20195ab9d1c1fc0dd28  stemming-quality.csv