Add diffusion Monte Carlo

[ohno]

Closes #34.

Summary

• add fixed-population pure diffusion Monte Carlo with systematic resampling
• provide seeded reproducibility, configurable equilibration, and optional initial walkers
• return the reference-energy history, final walkers, and sampling uncertainty
• validate ground-state energies for the harmonic oscillator and hydrogen atom

Testing

• julia --project=. --startup-file=no -e 'using Pkg; Pkg.test()'
• DOCUMENTER_LOCAL=true julia --project=docs --startup-file=no -e 'include("docs/make.jl")'
• Test.detect_ambiguities(TwoBody; recursive=true)

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