Paul's July 2026 updates

[joshkamm]

Summary

Branched from main, carries Paul's recent updates plus cleaned-up student contributions.

• Reworks integrals code paths: becke, cpu_util, cuda_util, gauss, integrals, integrals_aux
• Reworks libcintw: cintprep, cintwrapper
• libio overhaul: adds molden.cpp (new), rewrites read.cpp/write.cpp
• Bumps the vendored include/cint.h / cintwrapper.h from libcint 4.4.0 to 6.1.3 to match the libcint version actually linked on Athena and other recent environments (see commit 17ed9f8: "Note that cint.h is from a more recent libcint library.")

Notes

• Fast-forward from main, no divergence.
• CI has not yet run on this branch (push-only, not PR-triggered). This PR is the first CI signal.
• The libcint header bump touches the same area as open PRs Reorder cintw includes so linked libcint cint.h wins (transitional) #94 (cintw include ordering) and Supply CUDA math libs via CUDAToolkit imported targets #100 (CUDA math libs via CUDAToolkit) — worth checking for overlap before merging those.
• Anyone linking against an older libcint than what's now vendored here may see breakage; this is a known tradeoff, not an oversight.
• ZEST's paul_july_2026_updates branch depends on this branch merging first (ZEST's pixi dependency tracks SlaterGPU's main).

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