feat: add solve_poisson_robust (Split 1) and tests

[Ao-chuba]

@marco-2023 @PaulWAyers
This PR implements solve_poisson_robust() in src/grid/robust_poisson.py, introducing the Split-1-Plus-Solve pipeline for a more robust electrostatic potential calculation. Related to #288. The solver improves on the plain solve_poisson_bvp() by pre subtracting the sharp nuclear core density before passing the residual to the BVP solver.

Pipeline:

Total density rho(r)
      |
[SPLIT 1]  load pre-fitted Gaussian params via load_atomic_gaussian_params()
      |     subtract core density → smooth, residual
      |
[SOLVE]    solve_poisson_bvp on residual → phi_numerical
      |
Total Potential = phi_core (analytical) + phi_numerical

No new external dependencies introduced.

[Copilot]

Pull request overview

This PR adds a new “Split 1 + BVP” robust Poisson solver (solve_poisson_robust) that subtracts an analytical, pre-fitted Gaussian “core” density before solving the smoother residual numerically, then re-adds the analytical core potential.

Changes:

• Added solve_poisson_robust() implementing the Split-1-Plus-Solve pipeline for improved stability on cusp-like densities.
• Added a new test suite covering accuracy on Gaussian densities, stability on sharp/negative densities, and error handling for unsupported elements.
• Exposed the new solver via the package grid top-level import.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
src/grid/robust_poisson.py	Implements Split-1 core subtraction + solve_poisson_bvp residual solve and returns a total-potential callable.
src/grid/tests/test_robust_poisson.py	Adds tests validating correctness/stability and basic API behavior for the new solver.
src/grid/__init__.py	Re-exports the robust Poisson solver from the top-level package.

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[Ao-chuba]

the suggestions given by copilot was valid to some extend . and changes were made accordingly , do check in the updated pr
@PaulWAyers @marco-2023