Unify PySCF and GPU4PySCF nuclear-gradient computation

[Jens (JensWehner)]

• Add a shared feature_derivatives() implementation based on torch.autograd.grad().
• Add grid_derivative_block() to centralize slicing of grid-resolved derivatives.
  • Handles point-first layouts for grid coordinates and weights.
  • Handles point-last layouts for density, density gradient, and kinetic density.
• Add contract_veff_block() for the shared AO/XC derivative contractions.
  • Documents PySCF’s packed AO derivative layout.
  • Shares density, gradient, and kinetic-energy contractions between CPU and GPU paths.
• Add focused tests for feature differentiation, grid-block slicing, and effective-potential contraction against hardcoded reference values.
• Update the supported Pixi version from 0.75 to 0.78 and refresh the lockfile.

[Copilot]

Pull request overview

Unifies CPU and GPU nuclear-gradient calculations through shared PyTorch utilities and updates Pixi tooling.

Changes:

• Adds shared feature differentiation, grid slicing, and AO/XC contraction helpers.
• Refactors PySCF and GPU4PySCF gradient adapters to use shared logic.
• Adds focused tests and upgrades Pixi to 0.78.

Reviewed changes

Copilot reviewed 15 out of 16 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
skala/src/skala/pyscf/gradient_core.py	Adds shared gradient operations.
skala/src/skala/pyscf/gradients.py	Adopts shared CPU gradient logic.
skala/src/skala/gpu4pyscf/gradients.py	Adopts shared GPU gradient logic.
skala/tests/test_gradient_core.py	Tests shared gradient operations.
pixi.toml	Requires Pixi 0.78.
pixi.lock	Refreshes platform identifiers.
.github/workflows/skala-test.yml	Updates Pixi in runtime CI.
.github/workflows/pypi.yml	Updates Pixi in release CI.
.github/workflows/model-test.yml	Updates Pixi in model tests.
.github/workflows/model-examples.yml	Updates Pixi in model examples.
.github/workflows/model-benchmark.yml	Updates Pixi in benchmarks.
.github/workflows/lint.yml	Updates Pixi in lint CI.
.github/workflows/gauxc-test.yml	Updates Pixi in GauXC tests.
.github/workflows/examples.yml	Updates Pixi in example CI.
.github/workflows/docs.yml	Updates Pixi in documentation CI.
.github/workflows/benchmark-test.yml	Updates Pixi in benchmark tests.

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