Add material: Vespel SP-3 Polyimide

[scroberts]

Summary

Adds Vespel SP-3 Polyimide (material #64, 13th plastic) from the DuPont Vespel SP-3 Typical ISO Properties data sheet (VPE-A10869-00-B0614, 2014). Same base polyimide resin as the SP-1 entry added yesterday (#1), loaded with MoS₂ for low friction in vacuum/dry service.

Modeling calls (all documented in identification.notes)

• youngs_modulus = 3.275 GPa — the 23 °C ISO flexural modulus (ASTM D790). DuPont publishes no tensile/Young's modulus for the Vespel SP grades and designates flexural modulus as the modulus of elasticity — same call as the SP-1 entry.
• yield_strength null (no yield point; breaks at ~4.0 % elongation) → M4–M6 blank.
• poissons_ratio null — data sheet shows "—"; SP-1's 0.41 not transferred because the MoS₂ filler alters elastic response.
• compressive_strength = 127.6 MPa — stress at 10 % strain (ASTM D695); the material does not fracture in compression.
• shear_strength null — data sheet's ASTM D732 row is blank and a von Mises estimate does not apply (no yield point, ~4 % brittle-ish failure). Left null per the CLAUDE.md shear-derivation convention.
• fatigue_sn_curve empty — the data sheet lists axial/flexural fatigue endurance-limit rows but leaves every value blank.
• usable_temp_range −240 to 300 °C cites dupont-vespel-s-handbook (SP-family service guidance), not the SP-3 data sheet, which only reports data to 260 °C — noted explicitly.
• melting_point_tm / glass_transition_tg null — thermoset-like condensation polyimide, no melt or Tg below decomposition (>400 °C).
• specific_heat null → thermal_diffusivity null (would otherwise be computed from k/(ρ·Cp)).
• hardness null on all scales — the SP-3 data sheet publishes none.
• electrical_conductivity 0 / ref null — polyimide is an electrical insulator, nominally 0 % IACS, not a sourced measurement (per convention).
• Filler: the data sheet says only "a filled polymer"; the "15 wt% MoS₂" figure is attributed in notes to DuPont's broader Vespel SP literature, not this sheet.

Checklist

• python tools/validate.py passes locally (64/64)
• python tools/update_manifest.py run
• New reference dupont-vespel-sp3-datasheet added directly to references/index.json (admin-maintained, matching the SP-1 PR)
• Slug matches filename (vespel-sp3)
• All property values stored in canonical units
• lint_physics.py clean, check_docs.py OK

Materials added / changed

• materials/plastics/vespel-sp3.json (new)
• references/index.json — new dupont-vespel-sp3-datasheet entry
• materials/index.json — regenerated (63 → 64)
• README.md, SPEC.md — counts 63 → 64, Plastics 12 → 13

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