docs(tensors): add Tensors API map

[Robby955]

Motivation

Related to #1414, the tracking maps for the tensor API in #908 and the Lorentz tensor API in #886. Both issues are still unfilled templates, so the map supplies the requirement list they ask for. An API map records the implemented status and source location of each requirement, so the gap between a planned API and the current library stays visible in-tree.

Changes

Adds Physlib/Relativity/Tensors/API-map.yaml, with 22 requirements, 16 of them done. The completed entries cover TensorSpecies and the types Tensor, Pure and ComponentIdx; the group action, index permutation, products, contraction of a dual pair, evaluation, and the unit and metric tensors with raising and lowering; the index-notation elaborator and Tensorial; conjugation; and the species realLorentzTensor and complexLorentzTensor with the semilinear map between them.

Six are recorded as not done: neither Lorentz species is extended to a ConjTensorSpecies, so reality and Hermiticity cannot yet be stated for them; the metrics and units of the real species are still built from ContrMod and CoMod rather than as intertwiners of Vector and CoVector; three commutation lemmas for evalT are absent; there is a Euclidean Levi-Civita symbol but no Euclidean tensor; TensorSpecies carries no condition relating the metrics to the basis vectors; and the tensor-level epsilon-epsilon identities leviCivita_contract_self and leviCivita_contract_three are stated but carry sorry, so they are recorded the way solidSphere_inertiaTensor is in the rigid body map. The componentwise identities euclidLeviCivita_symbol_contract_zero, _one and _two are proved and stay under the done entry. Nothing outside the new file changes, and no Lean source is touched.

Checks

python3 scripts/api_map_linter.py --repo . passes with no missing files and no missing names, resolving 289 declarations against the Lean sources.

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[jstoobysmith]

Looks good - approved. Will merge shortly