Bump dashu-base/int/float/ratio/cmplx/macros and the dashu meta-crate from
0.5.0 to 0.5.1, and fold each crate's Unreleased changelog into 0.5.1.

Additive + fixes only (no breaking changes, MSRV unchanged at 1.68):
- base: bit()/encode() bug fixes (fuzz-found)
- int: to_f64 DoubleWord::MAX exactness, GCD scratchpad sizing
- float: Repr::new_const, log2; powi/to_f64/exp fixes
- ratio: from_str_expanded/from_str_decimal, Pow<isize>; GCD fix
- cmplx: CBig::from_parts_const
- macros: const cbig!

The InvalidSyntax variant and its downstream error-taxonomy refinement
remain on the develop branch (v1 line); master stays break-free vs 0.5.0.

Co-Authored-By: Claude <noreply@anthropic.com>