Fix polar harmonic annotations

[chenzc24]

Summary

• fix HD2/HD3 polar annotation magnitudes to read from harmonics_dbc
• correct HD2/HD3 phase bin indexing and keep the metrics box focused on HD2/HD3
• make analyze_spectrum_polar(harmonic=...) compute enough harmonics for the requested plot depth
• add regression tests for polar annotation magnitudes, phase indexing, and harmonic depth

Validation

• uv run --with pytest pytest tests/unit/spectrum/test_plot_spectrum_polar.py tests/unit/spectrum/test_extract_harmonic_powers.py tests/unit/spectrum/test_verify_compute_spectrum.py -q
• generated a polar smoke plot confirming HD4 markers remain on the plot while the summary stays HD2/HD3 only