Builder Pattern

A ballistic solution takes dozens of optional, interacting inputs. A flat solve function would have unusable arity and no place for cross-field validation, such as a zero distance without a velocity being meaningless. The builder lets callers name only what they know, defaults the rest, and validates the whole set before the solver runs.

The two stages also let high-fidelity branches ask "is this geometry complete?" without penalizing minimal builds: a sparse caller gets a well-formed solution, a fully described projectile unlocks the demanding models. See Builder Pattern for the builder surface.