Strong Unit Types

Inside the solver, quantities carry their engineering units as types, and each type does at least three jobs. It prevents mixing up arguments: two parameters that would both be plain numbers become distinct types, so passing them in the wrong order fails to compile. It converts implicitly within its category, so a call site states intent once in whatever unit reads naturally and the factor follows. And it otherwise behaves like its underlying value, at no runtime cost. Mixing categories is ill-formed: lengths, speeds, and angles stay distinct.

The motive is a class of bug with no test failure but a wrong trajectory: a unit mixup deep in the integrator, atmosphere, or spin corrections.

The types cost nothing at runtime and stop at the ABI boundary, where plain numeric values cross. Safety is recovered at the ergonomic layer by naming units in the builder setters, so a caller states what each number means. Direct C callers must get units right themselves.