Add Weinstein energy bounds

[ohno]

Summary

• add weinstein and WeinsteinBound for Rayleigh–Ritz results
• evaluate the energy variance from the continuous radial Hamiltonian action
• support non-relativistic kinetic, Laplacian, rest-energy, and radial potential terms
• document the bound, usage, and original reference

Impact

Users can calculate E ± σ without obtaining the artificial zero variance produced by projecting H² back into the finite variational basis.

Validation

• Pkg.test() — 557/557 tests passed
• complete Documenter build passed
• no method ambiguities detected
• the documented hydrogen interval contains the exact energy
• git diff --check

Closes #50

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