refactor(Electromagnetism): replace fieldStrengthMatrix with index-evaluation components

[zhikaip]

First of two PRs for the TODOs in Electromagnetism/Kinematics/FieldStrength.lean; the
follow-up fieldstrength_refactor removes the basis-representation lemmas.

Changes

• Components of the field strength are accessed as toField {A.toFieldStrength x | [μ] [ν]}ᵀ
  everywhere. fieldStrengthMatrix and its API are removed, and every lemma stated with it is
  restated in eval form, with fieldStrengthMatrix ↦ toFieldStrength_eval in the name.
• Two new lemmas, toFieldStrength_eval_eq_tensor_basis_repr and
  toFieldStrength_tensor_basis_repr_eq_eval, bridge eval components and the coefficients in
  Tensor.basis, the tensor library's canonical basis. They replace the role of the
  fieldStrengthMatrix bridge lemmas: KineticTerm expands F_{μν}F^{μν} as a sum over
  Tensor.basis coefficients and needs to turn each coefficient into a component, and
  antisymmetry, additivity and homogeneity of toFieldStrength are derived from their
  component versions through the same bridge.
• Tensors/Elab.lean: the bracket index syntax accepts a term instead of an ident, so
  concrete components such as [Sum.inl 0] [Sum.inr i] can be written. Existing uses are
  unaffected.
• Four files gain attribute [-simp] Fin.succAbove_zero (MagneticField, Boosts, KineticTerm,
  Hamiltonian); each fails without it.
• canonicalMomentum_eq is reproved via Lorentz.Vector.inner_eq_sum, and the two options
  in Vector/MinkowskiProduct.lean are dropped. With feat(Lorentz.Vector): mark Vector and CoVector implicit_reducible, add inner_eq_sum #1610 and fix(Tensors): rank the tensor action above the Tensorial action #1611 no
  respectTransparency option remains in the electromagnetism files.

Why

• Index evaluation is the tensor library's own notation for components; fieldStrengthMatrix
  was a second representation of the same thing with a duplicate API.
• Fin.succAbove_zero: plain simp rewrites Fin.succAbove 0 to Fin.succ inside the
  implicit colour index of evalT, after which no eval lemma matches. The tensor library
  disables it file-locally for the same reason; the attribute does not propagate.

replacements are mostly mechanical and AI seem to do a good job. I checked it to the best of my ability

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[zhikaip]

Btw I think toField is extremely confusing with all the field strengths EM fields etc. flying around, I'm thinking of changing it to something like toScalar. I can do that in a follow up PR if you agree

[jstoobysmith]

I think this would make sense. Sorry am at a conference, so haven't had time to look at this properly yet.

[jstoobysmith]

This all looks good to me - have approved