feat: add PyFock phase-one backend

[NCCU-Schultz-Lab]

Summary

Implements M-PYFOCK Phase 1 (PYF.2–PYF.6), establishing PyFock as QuantUI's first alternate quantum-chemistry engine for a guarded native-Windows-capable DFT subset.

• add the real PyFock Mol / Basis / DFT adapter and normalized EngineResult
• route single-point calculations through the shared engine registry while retaining PySCF as the default
• add capability-driven engine, calculation-type, method, basis, solvent, GPU, density-fitting, checkpoint, pre-opt, and SLURM gating
• persist/display engine provenance in results and logs
• add the [pyfock] extra plus a dedicated Windows/Python 3.11 real-calculation CI gate
• document installation, supported platforms, and engine limitations

Validated subset

PyFock Phase 1 intentionally supports:

• neutral, closed-shell singlets
• PBE single points
• def2-SVP and def2-TZVP
• always-on density fitting with def2-universal-jfit
• CPU execution

B3LYP/PBE0 remain gated because PyFock 0.1.7 does not yet implement the required exact exchange. Geometry optimization, solvent, GPU, checkpoints, SLURM, post-HF, and orbital analysis remain future phases.

Validation

• real PyFock/PySCF H₂O PBE/def2-SVP parity: passed <1e-5 Ha
• PyFock configured for spherical 5d/7f AOs to match PySCF's def2 convention
• focused regression set: 86 passed
• pre-commit: trailing whitespace, EOF, YAML, merge-conflict, large-file, debug, Black, and Ruff checks passed
• package build test passed

The dedicated native-Windows PyFock job added here is expected to provide the first Windows hardware/software gate on this PR.

Attribution

• Codex (GPT-6 Astra): engine adapter, dispatch, UI gating, tests, documentation, and PR preparation
• Jonathan Schultz: overall vision, planning, hardware verification, review, and orchestration