Add structural assertions for plot tests

[chenzc24]

Depends on #45.

Part of the remaining follow-up work for #36.

Summary:

• Add structural assertions to plot_spectrum tests so they verify rendered axes, lines, labels, markers, and annotations instead of only checking that PNG files exist.
• Strengthen sweep_performance_vs_osr plot tests by checking the main performance axis, legend, ENOB twinx axis, and auto-created slope subplot.
• Strengthen plot_residual_scatter tests by checking scatter collections, point counts, axis labels, pair mapping, and hidden unused subplots.

Notes:

• This is intentionally limited to common plot APIs and does not address the broader AOUT diagnostic plot files or slow-test audit yet.
• No product code changes.

Tests:

• uv run --with pytest pytest tests/unit/spectrum/test_plot_spectrum.py tests/unit/spectrum/test_sweep_performance_vs_osr.py tests/unit/dout/test_plot_residual_scatter.py -q
• uv run --with pytest pytest tests/unit/spectrum tests/unit/dout/test_plot_residual_scatter.py -q

[Arcadia-1]

Closing as superseded by #48, which was squash-merged into main and includes this stack layer's plot assertion changes.