[KIM][KiLCA] Carry the B-parallel vacuum drive through the periodic response

[krystophny]

Parent

#261

What to build

Add one usable B-parallel magnetic-compression path from the existing KiLCA vacuum response into the periodic KIM local-response contract. The mapping from KiLCA's field representation must preserve units, phase, signed mode convention, and normalization, and B-parallel must remain independent of the radial magnetic drive at the interface.

Periodic electrostatic KIM treats both magnetic components as prescribed drives. Their common shielding amplitude is applied later by QL-Balance.

Acceptance criteria

• The mapping from KiLCA Bp to physical B-parallel is derived and documented with units, basis, phase, and signed-mode conventions.
• Vacuum Br and B-parallel shapes are extracted and normalized at the same resonance with recorded provenance.
• The periodic request and result represent B-parallel independently of Br.
• Charge and parallel-current response columns for B-parallel are implemented for the supported collision and harmonic models.
• Scaling both magnetic drives by one complex scalar scales potential, electric fields, and species current linearly.
• Setting B-parallel to zero reproduces the pre-existing radial-drive response.
• A B-parallel-only analytic long-wavelength fixture checks sign, species charge, and phase.
• Production configuration cannot silently replace a requested B-parallel drive with zero.
• Unsupported harmonic/collision combinations fail during validation rather than omitting terms.

Blocked by

• [KIM] Make nonzero cyclotron-harmonic support honest and internally consistent #258
• [KIM] Return a state-isolated periodic local response #285