Rename vignetting transmission to illumination

[roytsmart]

Summary

Renames the optika.vignetting calibration member transmission → illumination.

The vignetting model represents the relative spatial response of the system, normalized to one at the center of the field of view. So transmission — which reads as an absolute transmitted fraction — was misleading now that absolute optical throughput lives in the (forthcoming) optika.radiometry effective-area model. illumination is the distilled form of the standard "relative illumination" term.

Renamed across:

• AbstractInterpolatedVignettingModel.illumination (abstract property)
• PolynomialVignettingModel.illumination (field), and its use in fit/plot/plot_residual
• the class docstrings, the example, and the tests

Testing

• pytest optika/vignetting/_vignetting_test.py — 26 passed; 100% coverage.
• black --check and ruff check clean.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.31%. Comparing base (c3d1e62) to head (a82b66d).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #159   +/-   ##
=======================================
  Coverage   99.31%   99.31%           
=======================================
  Files         114      114           
  Lines        5722     5722           
=======================================
  Hits         5683     5683           
  Misses         39       39           

Flag	Coverage Δ
unittests	99.31% <100.00%> (ø)

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