Add structural assertions for AOUT diagnostic plots

[chenzc24]

Depends on #46.

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

Summary:

• Add structural assertions to AOUT diagnostic plot tests that previously only checked generated PNG files.
• Verify PDF panels render KDE/Gaussian lines, labels, legends, stats text, and finite returned PDF arrays.
• Verify autocorrelation, error spectrum, envelope spectrum, phase-plane, and residual phase-plane plots render expected axes, artists, labels, and returned data shapes.

Notes:

• This PR is intentionally limited to six AOUT diagnostic plot tests.
• No product code changes.
• Slow-test audit remains separate follow-up work.

Tests:

• uv run --with pytest pytest tests/unit/aout/test_analyze_error_pdf.py tests/unit/aout/test_analyze_error_autocorrelation.py tests/unit/aout/test_analyze_error_spectrum.py tests/unit/aout/test_analyze_error_envelope_spectrum.py tests/unit/aout/test_analyze_phase_plane.py tests/unit/aout/test_analyze_error_phase_plane.py -q
• uv run --with pytest pytest tests/unit/aout -q

[Arcadia-1]

Closing as superseded by #48, which was squash-merged into main and includes this stack layer's AOUT plot assertion changes.