refactor(Tensors): rename Tensor.toField to Tensor.toScalar

[zhikaip]

Tensor.toField maps a rank-zero tensor to its underlying scalar in k. In the electromagnetism files it sits right next to toFieldStrength, for example toField {A.toFieldStrength x | [μ] [ν]}ᵀ, where "field" reads as a physical field. k is also only assumed to be a CommRing, not a field. This PR renames the map and its lemmas; no statements or proofs change.

Renamed (old toField → new toScalar)

• Relativity/Tensors/Basic.lean: toScalar (def), toScalar_default, toScalar_injective, toScalar_pure, toScalar_permT, toScalar_basis_default, toScalar_basis, toScalar_eq_repr, toScalar_equivariant, eq_smul_toScalar
• Relativity/Tensors/RealTensor/Basic.lean: contrT_toScalar
• Relativity/Tensors/RealTensor/Vector/Tensorial.lean: Vector.toScalar_eval_eval_eq_tensorProduct_repr
• Particles/SuperSymmetry/N1/Basic.lean: toScalar_conjScalar

No deprecated aliases are added.

Reviewer map

1. Tensors/Basic.lean: the definition and its core lemmas. Docstrings and the section heading now say "scalars" instead of "field".
2. The rest is a mechanical find-and-replace in the other Tensors files, the Electromagnetism files, N1 and API-map.yaml.

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[jstoobysmith]

Approved - makes sense and looks good