build, add Spack package recipe

[steven-varga]

Add Spack support so HPC users can install h5cpp-compiler via spack install h5cpp-compiler alongside their existing HDF5/MPI stacks.

Scope

• Add package.py recipe for h5cpp-compiler
• Scaffold custom Spack repo (repo.yaml, packages/)
• CI job that validates spack install on ubuntu-24.04
• README update with Spack instructions

Notes

• Spack LLVM package typically tracks upstream closely; LLVM 20 support likely already present.
• Aligns with HDF5/HPC user base where Spack is the dominant package manager.