docs(electromagnetism): add kinematics API map

[Robby955]

Motivation

Related to #1414. This adds an API map for the electromagnetic-potential API (the "API - Electromagnetic potential" issue). An API map records the implemented status and source location of each requirement, so the state of the API stays visible in-tree.

Changes

Adds Physlib/Electromagnetism/Kinematics/API-map.yaml.

The map records the electromagnetic-potential API: the ElectromagneticPotential structure with its additive-group and real-scalar-action and Lorentz-group-action API, its constructors (from a scalar potential, a vector potential, and a pair of non-relativistic potentials), the distributional counterpart DistElectromagneticPotential, and the derived electric field, magnetic field (as a vector and as an antisymmetric matrix), field-strength kinetic term, and Lagrangian, each in both the function and distributional settings. The one requirement that is not implemented, identifying the potential with a connection on a U(1)-gauge bundle, is marked not done with location N/A, since the library only remarks on it in documentation.

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]

Have we checked this reference? If so this looks good.

[Robby955]

Checked against the third edition: potentials are 6.2 and gauge transformations 6.3.

[jstoobysmith]

Approved - will merge shortly.