Add `RandomBinder`, the by-name sibling to `SweepBinder`, so a random
sweep (`RandomSpace`, C12.1) can be built by name against `param_space()`
instead of by positional `Vec<ParamRange>` slot order. By-name resolution
makes a same-kind transposition (e.g. swapping the I64 ranges for
`fast.length` and `slow.length`) structurally impossible — the failure
class `RandomSpace::new`'s positional validation passes silently.
Direct structural mirror of `SweepBinder`: `Composite::range` opens the
binder, `.range(name, ParamRange)` accumulates, `.sweep(count, seed, run)`
resolves the named ranges via the shared `resolve_into` (new `resolve_ranges`
caller) and runs the disjoint sweep. New `BindError::EmptyRange` mirrors
`EmptyAxis`; `ParamRange::is_empty` homes the non-empty invariant the named
layer pre-checks so `RandomSpace::new` cannot fail.
closes#79
Brummel
91ef69ed7b