fix(VariantScreen): use mapped coordinate mode for histogram

[bencap]

The Variant view histogram fell back to raw-mode tooltip labels because three things lined up: the coordinates prop defaulted to 'raw', the variant cache only ran parseScoresOrCounts (so parsedPostMappedHgvs* and translated_hgvs_p were never populated), and the fetch omitted include_post_mapped_hgvs plus the vep/clingen namespaces — so even a mapped-mode tooltip would have had nothing to render. Pass coordinates="mapped" on VariantScreen and route the fetch + parse through the same pipeline ScoreSetView uses.

Along the way, consolidate the variant-data plumbing that had drifted between the two screens:

• Extract histogramScoreSetVariantDataUrl and a small params/URL builder in api/mavedb/score-sets.ts; ScoreSetView and the histogram fetch now share one definition of the endpoint and namespace set.
• Add parseScoreSetVariantData in lib/variants.ts as the single entrypoint for the parseScoresOrCounts → parseSimpleCodingVariants → translateSimpleCodingVariants chain; the latter two are now internal.
• Rename getVariantScores → getHistogramVariantData to reflect that it returns full variant rows, not just scores.
• Type useVariantLookup's scores/variantScoreRow as Variant instead of ScoresOrCountsRow, removing the as any cast at the histogram :variants binding.
• Introduce ParsedSimpleDnaVariation/ParsedSimpleProteinVariation aliases so the residueType/origin fields added by parseSimpleCodingVariants are reflected in the type.