Integrate NIST PhysRefData / XCOM / FFAST — radiation length & X-ray attenuation

[gerchowl]

NIST X-ray databases: XCOM (photon cross-sections), FFAST (form factor / attenuation), Atomic Weights / Isotopic Compositions. US-gov, effectively public domain for redistribution; scalar fields py-mat keeps (radiation_length_cm, interaction_length_cm, mean_excitation_energy_eV) live here.

Note: tabular cross-section data goes to nucl-parquet, not py-mat (#157). Only ingest scalars into py-mat.

Reference: https://www.nist.gov/pml/xcom-photon-cross-sections-database, https://www.nist.gov/pml/x-ray-form-factor-attenuation-and-scattering-tables

[gerchowl]

Deferred to a future 4.x minor release per Phase 4 design review (3 parallel reviewers, May 2026). NIST PhysRefData / XCOM / FFAST — deferred to 4.x: radiation length tables are detector-physics specific. The optional [nuclear] extra (#157) already delegates this concern to nucl-parquet (~50 KB per element of XCOM data). When a consumer needs it, py-mat[nuclear] is the path; no need to bulk-import into core TOMLs.

Not closed — the issue stays open with the deferred-4.x label so it surfaces in future planning.