Add custom basis functions

[ohno]

Summary

• add CustomBasis for user-defined radial s-wave functions
• evaluate overlap, kinetic, rest-energy, and central-potential matrix elements by radial quadrature
• share the quadrature helper with custom-potential matrix elements
• add API documentation and comparisons with analytical Gaussian basis results

Impact

User-defined functions can now participate directly in Rayleigh–Ritz calculations when they satisfy the documented numerical requirements.

Validation

• Pkg.test() — 556/556 tests passed
• complete Documenter build passed
• no method ambiguities detected
• custom and analytical Gaussian calculations agree within 1.3e-12
• git diff --check

Closes #49

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This pull request was developed with assistance from Codex using GPT-5.6 Sol with High reasoning effort.