feat: generate FDTD materials from MaterialCards

[vvahidd]

Summary

• Treat resolved MaterialCards as the canonical FDTD material definitions.
• Serialize scalar, tabulated, Sellmeier, Drude, and Lorentz optical models to the FDTD schema while retaining the center-wavelength background index for sizing.
• Add strict schema validation, common lowercase material aliases, documentation, and adapter coverage.

Validation

• Full test suite: 1050 passed, 1 skipped.
• Changed-file pre-commit checks passed.
• A generated gpdk MMI config completed a fresh FDTD dry run successfully.