Implement point-group SCF density reconstruction

[benjipeng]

Summary

• retain exact point-group orbit metadata in KPointMesh and workload artifacts
• reconstruct scalar and collinear-spin SCF densities with device-resident FFT-grid permutations
• fail closed for unsupported point-group forces and stress
• update Fe q16 method evidence and DFT roadmap boundaries

Validation

• ruff check src
• API docs generation check
• 130 focused DFT regression tests
• tiny full/reduced cubic SCF parity
• Fe q16 matched oracle: 4.0505e-6 Ha/atom energy difference and 0.002563 moment difference