Consider adding random geometry generators: rand_sphere, rand_ball, rand_rot

[cortner]

Postponed from the test-utils consolidation survey. Several ACEsuit test suites carry small random-geometry generators. Worth considering for inclusion here:

• rand_sphere() — random point on the unit sphere
• rand_ball() — random point in the unit ball
• rand_rot() — random rotation matrix

Not in scope (package-specific): rand_config, make_batch, rand_graph, random_positions (Unitful/molecular), PBC config generators.

Decision deferred — capturing here so it isn't lost.