Assemble Hamiltonian by operator

[ohno]

Summary

• assemble the Rayleigh–Ritz Hamiltonian with the operator loop outermost
• verify that the assembled matrix equals the sum of its operator matrices
• acknowledge feedback from JuliaHEP Workshop 2025 participants

Impact

Matrix assembly reuses each operator across the basis-pair loop while preserving the numerical result.

Validation

• Pkg.test() — 552/552 tests passed
• git diff --check

Closes #35

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