feat: support parser rounding modes

[tisonkun]

Summary

• Add a public RoundMode with five non-redundant modes: Ceil, Floor, HalfCeil, HalfFloor, and HalfEven.
• Add a non-exhaustive ParseOptions with a public round_mode field and HalfCeil as its default.
• Add ByteSize::parse_with(input, options); FromStr delegates to it with ParseOptions::default(), preserving the existing ties-to-larger behavior for standard and Serde string parsing.
• Apply the unit multiplier first, then round exactly once to a whole byte without floating point; preserve enough discarded-digit state to distinguish exact ties from values just above a tie.
• Cover every supported mode with boundary tests, decimal and binary unit tests, overflow tests, and QuickCheck comparisons against exact u128 arithmetic.
• Document the contract in the API overview, README, and changelog.
• Verify with cargo x test, cargo x lint, and cargo x bench.

Design Notes

The rounding semantics are adapted from Jiff’s RoundMode, but the API does not copy all nine variants. ByteSize only accepts non-negative values, making Expand equivalent to Ceil, Trunc equivalent to Floor, HalfExpand equivalent to HalfCeil, and HalfTrunc equivalent to HalfFloor. The public enum keeps one direction-based name for each distinct behavior relevant to this domain.

The parsing API follows the cronexpr ParseOptions pattern: configurable behavior lives in a defaultable, non-exhaustive options type, while the ordinary parsing path delegates to the configurable entry point with default options. This gives future parser settings a compatible place to compose without adding more specialized parsing methods.

Fraction parsing always records whether any digit after the first discarded digit is nonzero. This removes both const-generic specialization and a per-digit mode branch; A/B benchmarks showed no material regression.