Add local CFT first-variation stack to PhyslibAlpha

[juanjfndz]

This PR moves the local classical field theory stack into PhyslibAlpha, following the discussion on the previous core Physlib PR about developing this coordinate-first material in the Alpha area first.

The development is based on the coordinate local setup used in Cortes--Haupt, arXiv:1612.03100v2: fields Space d -> EuclideanSpace ℝ (Fin m), coordinate-level jet points, local Lagrangians, total derivatives, and the first-variation / Euler--Lagrange pipeline.

The stack added here includes:

• coordinate-level jet points and jet fiber directions;
• regularity/support lemmas for jet-coordinate maps;
• total derivatives of jet-dependent functions;
• local Lagrangians and their coordinate derivatives;
• local action/criticality definitions;
• Euler--Lagrange operators;
• first-variation density, integration-by-parts, regularity, support, and criterion modules.

The underlying maintained AdmissibleVariation structure remains imported from Physlib; Alpha only adds the Euclidean component helper needed by this coordinate stack.

Validation run locally:

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

[github-actions]

Thank you for this PR, which will now be reviewed. Please
see our review guidelines
if you are not familiar with the process. You should expect a back and forth
with a reviewer before your PR is merged. See also that link for how to
add appropriate labels to your PR. The PR will also go through a number
of automated checks. You can learn more about these here,
including how to run them locally.

If you want to bring attention to this PR, please write a message on this
thread of the Lean Zulip.

[jstoobysmith]

Approved.