fix, register particle struct in bench-small_object workload

[steven-varga]

Summary

bench/workloads/small_object.cpp wrote an unregistered POD aggregate via h5::write(fd, "particle", p), tripping the static_assert in H5Dwrite.hpp:737 that requires storage_representation_v<T> to resolve to a supported representation.

CI doesn't enable H5CPP_BUILD_BENCH, so the failure only surfaced on a local cmake --build of everything with bench enabled.

Fix

Add H5CPP_REGISTER_STRUCT(Particle) after the type definition, mirroring the smart_ptr example pattern.