Add criticality transfer API to PhyslibAlpha

[juanjfndz]

This PR adds a small transfer API around IsEulerLagrangeEquivalent and TotalDivergenceEquivalence in the local Classical Field Theory Alpha stack.

It builds directly on #1307 and introduces:

• IsEulerLagrangeEquivalent.eulerLagrangeOp_eq, exposing the defining equality as a namespaced lemma;
• IsEulerLagrangeEquivalent.eulerLagrangeOp_eq_zero_iff, transferring Euler--Lagrange vanishing across equivalent lagrangians;
• directional criticality transfer lemmas from source to target and from target to source;
• the corresponding convenience wrappers for TotalDivergenceEquivalence.

This PR does not add new mathematical objects or symbolic calculus. It only packages the immediate consequences of the already-approved equivalence predicate, so downstream Alpha files can use named transfer lemmas rather than repeatedly destructing the same iff.

Validation:

• lake build PhyslibAlpha.ClassicalFieldTheory.Local.TotalDivergenceEquivalence
• lake build PhyslibAlpha
• lake exe runPhyslibAlphaLinters
• ./scripts/PhyslibAlpha/alphaPythonLinters.sh
• python3 ./scripts/PhyslibAlpha/alphaFileImports.py
• python3 ./scripts/PhyslibAlpha/noAlphaImports.py

[github-actions]

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[jstoobysmith]

Approved. Will merge now.