Test coverage gaps: AMICA.save/load and plot_components

[neuromechanist]

Surfaced during Phase 1 review (epic #9, PR #14).

• AMICA.save/AMICA.load (pyAMICA/amica.py:244-297) do a real torch.save/load state_dict roundtrip but have zero test coverage. The old AMICATorch.save/load test was deleted (those methods no longer exist); coverage moved to the wrapper and was never re-added.
• amica_viz.plot_components (pyAMICA/amica_viz.py:84) has a current signature (results_dir, data=None, ...) -> None and is fully untested since the stale test_visualization was deleted.

Add real (non-mock) tests: save/load a fitted model to tmp_path and assert roundtrip equality of W/A; render plot_components from a real saved results directory and assert files are written.

[neuromechanist]

The save/load half of this issue is now covered by #44 (merged): real fit -> save -> load -> transform round-trip plus unfitted/degenerate/non-finite/snapshot guard tests (test_amica_ng_wrapper.py, test_ng_backend.py::test_state_dict_*).

Remaining under this issue: amica_viz.plot_components test coverage. Keeping open for that.