Add optika.radiometry with an effective-area model

[roytsmart]

Summary

Introduces optika.radiometry, a subpackage for radiometric models of an optical system, starting with effective area.

• AbstractEffectiveAreaModel — interface mapping wavelength → effective area (__call__(wavelength) -> area).
• InterpolatedEffectiveAreaModel — holds calibration wavelength/area arrays and linearly interpolates between them via na.interp.

Linear interpolation (rather than a polynomial fit, as used by optika.distortion/optika.vignetting) is deliberate: a real effective-area curve has sharp features — absorption edges, filter cutoffs, multilayer reflectivity peaks — that a polynomial fits poorly.

optika.radiometry is intended as the home for the other radiometric factors of the linear forward model (energy-per-photon, voxel solid-angle/volume, etc.) as they're migrated in.

Testing

• pytest optika/radiometry/_effective_area_test.py — 7 passed; 100% coverage.
• The docstring example renders headless; black --check and ruff check clean.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.32%. Comparing base (c3d1e62) to head (a15bfbc).

Additional details and impacted files

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  Coverage   99.31%   99.32%           
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  Files         114      117    +3     
  Lines        5722     5771   +49     
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+ Hits         5683     5732   +49     
  Misses         39       39           

Flag	Coverage Δ
unittests	99.32% <100.00%> (+<0.01%)	⬆️

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