Define floatmin, floatmax, maxintfloat and issubnormal for Dual instances

[devmotion]

Fixes #852.

Following JuliaLang/julia#63451, typemin, typemax, floatmin, floatmax and maxintfloat accept instances as well as types. Base only forwards instances of AbstractFloat to the type methods, so floatmin(::Dual), floatmax(::Dual) and maxintfloat(::Dual) threw a MethodError. So did maxintfloat(::Type{<:Dual}) and issubnormal(::Dual).

These functions now return the primal, which is what eps already did, and they're defined together with eps in one loop. Before, typemin/typemax/floatmin/floatmax returned a Dual with zero partials, which only added overhead and could produce NaNs.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.21%. Comparing base (a3c0f4f) to head (b0d87da).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #855      +/-   ##
==========================================
- Coverage   91.23%   91.21%   -0.02%     
==========================================
  Files          11       11              
  Lines        1072     1070       -2     
==========================================
- Hits          978      976       -2     
  Misses         94       94              

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