ball_float_checked Design

BallFloatResult closes checked interval construction and arithmetic over Result[BallFloat, ArithmeticError]. Invalid bounds, non-finite exact sources, and checked arithmetic failures enter the error branch; later methods short-circuit. bind may introduce a failure and map cannot.

The wrapper neither stores decorations nor BallFlags, and it does not add an interval algorithm. Bare set arithmetic is delegated to ball_float; decorated operations and context status remain explicit APIs there. Conservative interval results such as Entire are valid successes, not wrapper errors.

Wrapper composition adds O(1) work beyond interval arithmetic. Entire remains success because loss of tightness is not the same as failure to enclose; making it an error would break valid interval-domain semantics.