feat(tensors): prove the Levi-Civita contraction identities

[Robby955]

Proves the two remaining epsilon-epsilon identities for the Lorentzian Levi-Civita tensor,
leviCivita_contract_self and leviCivita_contract_three, left as stated goals in #1348. Both
statements are unchanged.

The route is through components. Both sides are compared under (Tensor.basis _).repr and the
contractions collapse to finite sums over Fin 1 ⊕ Fin 3, where the product of two symbols is a
generalizedKroneckerDelta and the contraction machinery from #1335 gives the 24 and the 6. The
sign comes out of the metric: when the symbol is nonzero the index map is injective, hence a
bijection on a four-element type, so the four diagonal metric factors multiply to the determinant,
which is minus one in this signature.

Two component lemmas that did not exist before carry the argument and may be useful beyond it:
toDualMapAtIndex_basis_repr, the basis representation of index lowering, and
unitTensor_down_repr, the basis representation of the unit tensor of color down. The rest of the
new material is rank-four bookkeeping for this file.

The proofs elaborate within the default heartbeat budget, so no maxHeartbeats option is needed.
The nolint checkType on leviCivita_contract_self stays: that linter re-typechecks the statement,
which this change does not alter, and its comment now names the current toolchain. The Tensors API
map entry for these identities is flipped to done with a real location.

One question on placement. The two component lemmas would sit naturally next to their definitions,
toDualMapAtIndex_basis_repr in Dual.lean in particular, rather than in this file. I kept
everything local so the diff stays reviewable, and can move them wherever you prefer, here or in a
follow-up.

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[jstoobysmith]

Will try and give a proper review tomorrow, but I think this could do with lemmas moved around bit. For example there are some IsReindexing lemmas here which should likely be in the corresponding file not here

[jstoobysmith]

Some more things to move.

There are also lots of small definitions/lemmas that are:

1. Either not needed,
2. Can be reframed in terms of useful physics API.

[jstoobysmith]

This should be moved to the unitTensor file

[jstoobysmith]

Should be moved to the minkowskiMatrix file

[jstoobysmith]

This is not likely needed as a sperate lemma, similar with a lot of otherr small definitions here.

[jstoobysmith]

MOve e.g. to the metricTensor file

[jstoobysmith]

Move to crossToSlot file.

[jstoobysmith]

awaiting-author

[jstoobysmith]

(May also help to split this PR up into smaller bits each touching a single file)

[Robby955]

Thanks for the review. I have reorganized the proof around the owning APIs in ab91a437.

• The crossToEnd and crossToSlot component formulas now live in dedicated RealTensor/Contraction modules.
• Generic metric and unit component bridges now live in MetricTensor.lean and UnitTensor.lean; their Minkowski and Kronecker-delta specializations live in RealTensor/Metrics/Basic.lean and the new RealTensor/Units/Basic.lean.
• The diagonal metric product has been generalized to arbitrary spatial dimension and moved to MinkowskiMatrix.lean.
• The one-use sum-reindexing lemma was removed. The reusable free-last generalized Kronecker-delta contraction is now in KroneckerDelta/Contraction.lean.
• The two Lorentzian identities and their private component proof now live in LeviCivita/Contractions.lean; LeviCivita/Basic.lean is again limited to the definition, components, and antisymmetry.

The exact one-sum and Kronecker/Minkowski formulas are specific to the real Lorentz bases. Putting those statements directly in the generic tensor files would either create an import cycle or require basis/contraction compatibility not present in TensorSpecies, so the generic bridges and real specializations are separated.

The two headline theorem statements are unchanged. The targeted build, full lake build Physlib, changed-file style checks, API-map check, and declaration linters pass.

-awaiting-author