Add grazing-incidence soft X-ray spectrometer tutorial

[jacobdparker]

Summary

Adds a new docs tutorial, docs/tutorials/grazing_spectrometer.ipynb, that designs a slitless grazing-incidence soft X-ray spectrometer end-to-end in optika:

• a Wolter Type-I telescope — paraboloid (ParabolicSag) + confocal hyperboloid (ConicSag, k < −1), illuminated at large radius for grazing incidence;
• a 5000 line/mm transmission grating (ConstantRulingSpacing) placed in the converging beam; and
• a 2k × 4k CMOS detector (10 µm pixels) that records the zero-order direct image and the first-order spectrum side by side.

The design is driven by a 1 arcsec/pixel spatial plate scale (→ f ≈ 2.06 m) and a 15–20 Å first-order range. It's also a workout for parts of optika that hadn't been exercised before: grazing-incidence conic optics and transmission gratings.

Sections

• Focal length from plate scale; aperture → grazing angle (≈0.5°)
• Paraboloid and confocal-hyperboloid construction (with the subtlety that the hyperboloid's far focus must match the parabola's true focus at F_p − f_p for zero on-axis aberration)
• Transmission grating + dispersion check (≈0.86 mm/Å, zero order on-chip)
• Layout in the cross-dispersion plane, plus a dispersion-plane layout tracing m=0 and m=1 to show the spectral mapping onto the detector
• Spot diagrams (on-axis near-perfect; off-axis coma sets the usable field)
• Spatial plate-scale check
• Au/Ir grazing reflectivity sidebar (CXRO optical constants)

Notes for reviewers

• Independent of the in-flight FZP tutorial branch; based directly on main.
• Notebook executes end-to-end against main (jupyter nbconvert --execute) and outputs are stripped (nbstripout).
• Registered in the docs toctree (docs/index.rst).

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.32%. Comparing base (44177ca) to head (042318c).

Additional details and impacted files

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Flag	Coverage Δ
unittests	99.32% <ø> (ø)

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