[#11]:svarga:refactor, expand attribute type coverage to match full type engine support

[steven-varga]

Summary

• Adds explicit impl::data() and impl::size() overloads for std::array<T,N> in H5Mstl.hpp so awrite creates a correct rank-1 N-element attribute (previously fell through to the scalar POD path)
• Adds aread<A> overload gated on meta::is_array for clean std::array readback; guards arithmetic branch with !meta::is_array to prevent ambiguity
• New test H5Aall.cpp: 6 test cases, 39 assertions covering arithmetic scalars, std::string, vector<T>, array<T,N>, initializer_list, and ds[name] = value operator

Test plan

• ctest -R H5Aall — 6 test cases, 39 assertions pass
• std::array<int,4>, <float,3>, <double,2> roundtrips all pass
• CI matrix green