Add scalar frequency-dependent permittivity models

[hughcars]

Summary

Adds a scalar frequency-dependent Permittivity model for driven and 3D eigenmode simulations:

"Permittivity":
{
  "HighFrequency": 2.08,
  "Terms":
  [
    {
      "Type": "Drude",
      "PlasmaFrequency": 1.0,
      "CollisionFrequency": 0.1
    }
  ]
}

Supported additive terms are:

• Drude
• Debye
• Lorentz
• explicit pole-residue
• Djordjevic–Sarkar

Legacy scalar and principal-axis Permittivity inputs remain unchanged.

Frequency-dependent scalar coefficients reuse one unit vector-mass operator per material support. The high-frequency permittivity remains in the ordinary mass matrix, exact zero poles are folded into conductivity, and nonlinear terms are evaluated directly in the frequency-dependent operator.

The change also adds documentation and a Drude cylindrical-cavity example with serial and parallel regression coverage.

Validation

• Spack build/install with test dependencies and -j4 passed
• Non-regression, non-long CTest matrix passed: 185 tests passed and one expected source-schema test skipped
• Eight-rank cylinder_cavity_drude and existing floquet_lumped regressions passed locally
• Source schema validation, full documentation build, formatting, and diff checks passed
• Full standard CI matrix passed across Linux, macOS, Spack, GPU, containers, documentation, style, and CodeQL
• Long tests were not requested or run
• Final Palace-specific review: no findings