docs(classical-field): add gauge theory API map

[Robby955]

Motivation

Related to #1414, and a tracking map for the gauge-theory API in #859. An API map records the implemented status and source location of each requirement for an API, so the gap between a planned API and the current library stays visible in-tree.

Changes

Adds Physlib/ClassicalFieldTheory/GaugeTheory/API-map.yaml.

The library has no general or non-abelian gauge-theory framework yet, so those requirements (an abstract structure group, the connection, the covariant derivative, Yang-Mills curvature and action, Wilson lines, and a principal-bundle formulation) are marked not done with location N/A. The completed requirements point at the two concrete pieces that do exist: the abelian U(1) case in Physlib/Electromagnetism/Kinematics (potential, field strength, the gauge transformation and its invariance, and equivariance under the Lorentz action), and the Standard Model gauge group in Physlib/Particles/StandardModel/Basic.lean.

Testing

Ran the API-map linter:

python3 scripts/api_map_linter.py --repo .

The map passes: every declared source file and name resolves, and no completed entry is missing a location.

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[jstoobysmith]

Approved - many thanks