Five bite-sized tasks for the RandomSpace cut: (1) the Space trait +
behaviour-preserving generalization of sweep/sweep_with_threads, (2) typed
ParamRange, (3) RandomSpace::new validation + three SweepError variants,
(4) SplitMix64 -> pub(crate) + the seeded sampler, (5) public exports +
random-sweep integration. Each task is RED-first with single-substring test
filters; the grid suite is the C1 behaviour-preservation regression guard.
refs #52
