Improve quantity serialization, typing, and derived arithmetic

[nodivbyzero]

Description

• Round JSON and text scalar output to 15 significant digits.
• Add unit-specific JSON marshaling with MarshalJSONIn.
• Constrain dimensions and expose custom unit conversion methods.
• Add duration interop and typed derived operations.
• Expand README guidance on semantics and custom dimensions.
• Strengthen tests with fuzzing, custom-unit coverage, and round trips.
• Make benchmarks retain meaningful loop results.