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.
Prepare pkg/vector for a future refactoring of internal/ai/face without
touching the face package yet. Normalize scales a vector to unit length
(L2) in place and leaves a zero vector unchanged; Normalized returns a
normalized copy. Centroid returns the element-wise mean of a set of
vectors, ignoring entries whose length differs from the first and
averaging over the vectors actually included. Together with the existing
EuclideanDist, EuclideanNorm, and NewVector helpers these cover the math
currently hand-rolled in the face embeddings code. Fully covered by
tests (100% of the new functions).
