Constrain spectrum MaxSpur annotations

[chenzc24]

Summary

• Hide MaxSpur marker/text when the spur is outside the current plot viewport instead of forcing a misleading bottom-edge label.
• Keep in-range MaxSpur labels inside the axes after caller x/y limit changes, including edge and high-spur cases.
• Apply the same behavior to the Virtuoso spectrum plotter and clip harmonic labels to the axes.

Tests

• uv run --with pytest pytest tests/unit/spectrum/test_plot_spectrum.py -q
• uv run python src/adctoolbox/examples/03_generate_signals/exp_g01_generate_signal_demo.py