Add finite-displacement periodic phonons

[benjipeng]

Summary

• add explicit symmetry-reduced central-displacement planning and source-bound force sample identities
• add raw reciprocity and acoustic-sum-rule diagnostics without silently correcting force constants
• add mass-weighted Gamma-point modes, signed frequencies, acoustic translation overlap, and displacement convergence
• add atomic no-pickle partial-sample persistence with exact restart equivalence
• add a bounded 2-atom Silicon PBE/GTH-q4 material gate using eight SCFs

Verification

• 9 focused phonon core, persistence, restart, evaluator, and Silicon dry-plan tests passed
• Silicon material gate verified all 6 locked gates without ASR correction
• optical mean 511.175 cm^-1; reference-context error 4.999 cm^-1
• 0.02 to 0.01 bohr maximum drift 2.379 cm^-1
• fresh displacement-set walls 11.850 and 11.868 s; peak temporary 23,820,912 bytes
• ruff check src tests scripts
• static API documentation generation (68 pages)
• canonical site documentation sync check

Boundary

This closes the bounded Gamma-point phase. It does not claim phonon dispersion, supercell range convergence, LO-TO splitting, Raman/IR, or electron-phonon support.