Bump dashu-float, dashu-cmplx, and the dashu meta-crate from 0.5.1 to 0.5.2,
and fold each crate's Unreleased changelog into 0.5.2. dashu-base/int/ratio/macros
stay at 0.5.1 (no changes this cycle); their ^0.5.1 requirement already admits 0.5.2.

Additive + fixes + one documented behavior change (no breaking changes, MSRV
unchanged at 1.68):
- float: FBig::e / Context::e, fma, ulp_lb, addsub_{vv,vr,rv,rr}; mul/sqr/cubic
  now strictly correctly rounded; -0 rendering; x +/- 0 now rounds to context precision
- cmplx: CBig::fma / Context::fma; mul/div route cross-products through real FMA

The develop-side breaking change (R: ErrorBounds bound on the complex
transcendentals) remains on the develop branch (v1 line); master stays break-free
vs 0.5.1.

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