Apply parity boundary conditions

[ohno]

Summary

• apply the radial parity condition at the origin for central differences
• use the even-l Neumann condition to eliminate the excluded origin value
• preserve the odd-l zero-origin condition
• add direct stencil tests and concise boundary documentation

Impact

Central finite differences now distinguish ψ(-r) = ψ(r) from ψ(-r) = -ψ(r) while retaining the origin-free radial grid.

Validation

• Pkg.test() — 555/555 tests passed
• complete Documenter build passed
• existing hydrogen eigenvalues are preserved
• git diff --check

Closes #5

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