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.
CosineDist now returns a true cosine distance (1 - similarity), so
identical vectors yield 0; the previous similarity behavior moves to a
new CosineSimilarity function/method. GeometricMean no longer treats a
zero element as an uninitialized accumulator: zeros now correctly yield
0 and negative inputs return NaN. NewVector copies float64 and Vector
inputs so the result is independent of the source slice, matching the
other converters. Norm uses absolute values so odd powers (L1) stay
well-defined for negative components, variance is simplified, and the
numeric error paths return NaN instead of the Epsilon sentinel. Add
focused tests for the mean, distance, norm, and copy helpers.
