Two geometries

Add distance, a stateless command with metric, a, and b. Euclidean distance is the square root of the sum of squared differences. Cosine distance is 1 - dot(a,b)/(norm(a)*norm(b)).

Round the result to six decimal places and serialize integers without .0. Cosine distance for a zero vector is undefined; return the exact error in the tests. Validate dimensions and finite numbers too.

• Both metrics follow their mathematical definitions.
• Numeric formatting is portable across languages.
• Errors never emit NaN or infinity in JSON.