Add material: Vespel SP-1 Polyimide

[scroberts]

Unfilled DuPont Vespel SP-1, sourced primarily from the DuPont Vespel S Line Design Handbook (with MatWeb for the Rockwell E hardness row).

Notable modeling calls (all documented in the material notes):

• youngs_modulus stores the ISO flexural modulus (3.10 GPa) - DuPont publishes no tensile modulus for SP-1 and treats flexural modulus as the modulus of elasticity for this grade.
• yield_strength null (no yield point published) -> M4-M6 blank.
• melting_point_tm and glass_transition_tg both null - Vespel SP is thermoset-like with no melt or Tg below decomposition (>400 C).
• fracture_toughness / hardness_vickers null (not published).
• 2-point R=-1 Axial fatigue curve from Handbook Table 1 + Fig 29.

Regenerated materials/index.json; bumped README/SPEC counts 62 -> 63. validate.py 63/63, lint_physics.py clean, check_docs.py OK.

Claude-Session: https://claude.ai/code/session_011EH3uenu7WNdfpvxoEjU4s

Summary

Checklist

• python tools/validate.py passes locally
• python tools/update_manifest.py run if any material files were added or removed
• New references either exist in references/index.json or are embedded under new_references in the material JSON (admin: run python tools/import_new_refs.py --write after merge)
• Slug matches filename (identification.slug == filename without .json)
• All property values stored in canonical units (GPa, MPa, g/cm³, °C, µm/m·K, W/m·K, J/kg·K, % IACS, HV)

Materials added / changed