Error handling rules

All core numeric types avoid exceptions.

Err

• None: value is valid
• Invalid: parse failure / invalid input
• Overflow: exceeds limits or internal operation overflow
• DivByZero: division by zero

Propagation

• If an operand is in error state, operators return that operand (first error wins).
• Comparisons on invalid values are not meaningful; prefer ok() checks before comparing.

Conversions

• Numeric128::checked() returns std::expected<Numeric128, Err>.
• Numeric::parse_checked() and Numeric::from_int64_checked() return std::expected<Numeric, Err>.