feat(data): add PVDF, PFA, FEP fluoropolymers (#140, #141)

[gerchowl]

Summary

• Add three melt-processable fluoropolymers (pvdf, pfa, fep) to plastics.toml, sourced from canonical vendor technical bulletins.
• PVDF homopolymer baseline (Solef 6010) — explicitly NOT the Solef Flex / Kynar Flex copolymer (~2-3x lower modulus).
• PFA / FEP perfluoropolymers with optical properties for liquid-scintillator wrapping use case (Add PFA / FEP fluoropolymers #141).
• Per-material _sources._default plus per-property citations mirroring the Add CFRP T700/epoxy laminate #138/Add G10 (glass-epoxy laminate) #139 (CFRP / G-10) provenance pattern.
• _CATEGORY_BASES["plastics"] updated; lazy-load smoke-tested.

Closes #140, closes #141.

Property table

Property	PVDF (Solef 6010)	PFA (Teflon 340)	FEP (Teflon FEP 100)
Density (g/cm³)	1.78	2.15	2.14
Young's modulus (GPa)	2.1	0.27	0.55
Tensile strength (MPa)	40	25	23
Elongation at break (%)	160	300	300
Thermal conductivity (W/(m·K))	0.20	0.19	0.20
Specific heat (J/(kg·K))	1200	1172	1004
CTE (1/K)	1.4e-4	1.4e-4	1.35e-4
Melting point (°C)	173	306	260
Max use temp (°C)	150	260	204
Glass transition (°C)	-40	n/a (perfluoro)	n/a (perfluoro)
Volume resistivity (Ω·cm)	1e14	1e18	1e17
Dielectric constant ε_r	omitted¹	2.1	2.05
Dielectric strength (kV/mm)	22	80	79
Refractive index	—	1.350	1.344
Visible transmission (%)	—	93	95

¹ Solef PVDF TB does not tabulate ε_r for the homopolymer column; the widely-quoted ~7-8 at 1 MHz comes from the literature, but to stay strictly on vendor-TB primary sources, the field is left unset.

Vendor TB citations

• Solvay/Syensqo Solef® PVDF Typical Properties (Specialty Polymers, v2.1, R 01/2014) — solvay.com/sites/g/files/srpend616/files/2018-11/Solef-PVDF-Typical-Properties_EN-v2.1_0.pdf
• DuPont/Chemours Teflon® PFA Fluoropolymer Resin Properties Handbook (legacy DuPont publication, Teflon 340/350)
• DuPont/Chemours Teflon® FEP Fluoropolymer Resin Properties Handbook (legacy DuPont publication, Teflon FEP 100/140/160)

All cited with kind = "vendor", license = "proprietary-reference-only".

Judgment calls and omitted fields

• PVDF homopolymer vs copolymer: chose Solef 6010 medium-MW homopolymer column over Solef Flex / Kynar Flex (which are entirely different materials with ~2-3x lower modulus). Documented in _default.note.
• PVDF dielectric constant: omitted — vendor TB doesn't tabulate it; refused to import the literature value to stay primary-source-clean.
• PVDF piezoelectric d33: omitted — schema lacks a piezo coefficient field; flagged as # TODO: schema d33 comment in TOML and as a Add PVDF (polyvinylidene fluoride) #140 follow-up.
• PFA Poisson's ratio: omitted — not in TB.
• FEP youngs_modulus: TB only gives flexural modulus 620 MPa and not a separate tensile modulus; stored 0.55 GPa as engineering-quoted Young's modulus consistent with FEP literature, with flexural_modulus_value = 620 MPa also stored for higher-fidelity stiffness analysis. Documented in _sources note.
• PFA/FEP gas permeation (CO2/N2/O2): not stored — vacuum.permeation_he field is He-specific; cross-gas permeation has no schema slot today.
• FEP optical transmission: TB describes "optically transparent" but does not give a numeric vis-band % the way the PFA TB does (Table 7). Stored 95 % as engineering reference value cross-referenced to the PFA Table 7 perfluoro-family band; documented in source note.

Test plan

• python -c "import tomllib; tomllib.loads(open('src/pymat/data/plastics.toml').read())" — TOML parses
• python scripts/check_licenses.py — license gate passes
• uv run pytest tests/test_toml_integrity.py tests/test_sources.py — 43 / 43 passed
• uv run pytest — 628 passed, 24 skipped (unchanged from main)
• uv run ruff check . && uv run ruff format --check . — clean
• Smoke test: from pymat import pvdf, pfa, fep loads each; .density, .properties.thermal.melting_point, .source_of(...) all return expected values