Split the catch-all values.go into one file per concept, each mirrored
by its test: distance.go, norm.go, stats.go, product.go, centroid.go,
plus the mean methods folded into mean.go and Copy/Dim/Sum into
vector.go. Remove values.go, values_test.go, and values_more_test.go so
functionality and tests live where developers expect them.
Hoist the two 512-dimensional face embeddings shared by the distance,
norm, and cosine tests into fixtures_test.go, removing the previous
triplication, and decompose the monolithic TestVector into per-concept
tests. Close pre-existing coverage gaps in the integer converters and
the GeometricMean/HarmonicMean method wrappers, bringing the package to
100% statement coverage. Pure code movement; no behavior change.
