[tlwillke]

Fixes #779.

NVQParameters.idStringFor passed two arguments to a three-placeholder format string ("NVQ_%s_%d_%s"), so it threw MissingFormatArgumentException. The trailing _%s is removed, so the cache key is now NVQ_<dataset>_<nSubVectors>, matching the PQ key's style.

Nothing constructs NVQParameters today, so behavior doesn't change; this removes a latent failure for when NVQ is wired up as a compressor option.

Testing: adds CompressorParametersTest, which checks the NVQ cache key.

[github-actions]

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