test(build-machine): add machine build regression coverage

[georgeholt1]

• Adds a new freegsnke/tests/test_build_machine.py test module covering example machine construction, path vs direct data inputs, optional defaults, validation, copy behavior, plotting smoke coverage, and probe attachment.
• Adds a stored numerical baseline for the example machine R/M matrices and representative geometry.
• Adds compact in-memory machine fixtures covering single active coils, circuit active coils, scalar passives, polygon passives, direct magnetic probe data, and Latin Hypercube passive refinement.