Added SequentialSystem.vignetting(), a method to fit a polynomial vignetting model

[roytsmart]

Stacked on #181 (targets feature/system-distortion; will retarget to main when that merges).

Summary

• Add AbstractSequentialSystem.vignetting(), the vignetting counterpart of distortion(): the relative illumination at each scene coordinate is estimated from the fraction of unvignetted rays in the pupil, normalized to its maximum over the field of view, and fit with an optika.radiometry.PolynomialVignettingModel.
• Same interface as distortion(): custom wavelength/field/pupil grids (traced with rayfunction(), so grids left as None inherit the corresponding component of grid_input), normalized_field/normalized_pupil flags, polynomial degree, and a clear ValueError when the wavelength grid has no logical axis.
• The grid resolution and axis normalization shared between the two methods is factored into a private _rayfunction_and_axes() helper instead of duplicating ~30 lines.

Tests

test_vignetting mirrors test_distortion: parametrized over both grid sources and two polynomial degrees, asserting the model type, degree, the scalar-wavelength error branch, and that the fitted illumination calibration points lie in [0, 1].

pytest optika/systems/_sequential_test.py: 312 passed; black --check and ruff check clean.

🤖 Generated with Claude Code

https://claude.ai/code/session_01Lm9SxSpyNg9hx8dNL9pUXc

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.37%. Comparing base (c6a1b77) to head (7c291c8).

Additional details and impacted files

@@                      Coverage Diff                      @@
##           feature/system-distortion     #183      +/-   ##
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- Coverage                      99.37%   99.37%   -0.01%     
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  Files                            116      116              
  Lines                           6230     6204      -26     
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- Hits                            6191     6165      -26     
  Misses                            39       39              

Flag	Coverage Δ
unittests	99.37% <100.00%> (-0.01%)	⬇️

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