refactor(FieldStrength): remove basis-representation lemmas

[zhikaip]

Resolves the remaining TODO in Electromagnetism/Kinematics/FieldStrength.lean, following #1614.

Changes

• Removed toTensor_toFieldStrength_basis_repr, toFieldStrength_tensor_basis_eq_basis and toFieldStrength_eval_eq_basis_repr, the lemmas about the Mathlib product basis basis.tensorProduct basis on Vector d ⊗ Vector d, together with the TODO.
• What was general about them moves to the tensor library, for any F : Vector d ⊗ Vector d (RealTensor/Vector/Tensorial.lean): Lorentz.Vector.toField_eval_eval_eq_tensorProduct_reprandLorentz.Vector.tensor_basis_repr_toTensor_prod_apply`.
• toFieldStrength_antisymmetric, toFieldStrength_add and toFieldStrength_smul are now derived from their _eval component versions through toFieldStrength_tensor_basis_repr_eq_eval, rather than the other way round. The Tensor.basis bridge lemmas from refactor(Electromagnetism): replace fieldStrengthMatrix with index-evaluation components #1614 stay.
• golfed some proofs where appropriate

Why

After #1614 these lemmas had a single consumer each inside FieldStrength.lean, and nothing
about them was specific to the field strength.

Additional Effect of the series (#1610, #1611, #1614, this PR)

Two root causes fixed rather than worked around: Lorentz.Vector/CoVector are implicit_reducible, and the tensor action outranks the Tensorial transport action, so rw/simp match again where the respectTransparency option was compensating. One duplicate representation removed: field-strength components go through index evaluation only.

Counts against master before #1610:
set_option backward.isDefEq.respectTransparency false 553 → 508 library-wide
(Electromagnetism 21 → 11, Relativity/Tensors 107 → 72);

the files touched by the series have none. The remaining 72 in the tensor library were checked with scripts/rm_set_option.py and are needed for unrelated reasons.

mostly prepared by claude, supervised and checked by myself

[github-actions]

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[nateabr]

claim

[github-actions]

Review claim by @nateabr completed — thanks for the review.

[nateabr]

Look good to me, one very minor comment