Add a Glass refractive material with Sellmeier dispersion

[jacobdparker]

What

Adds optika.materials.Glass, the first surface material whose index of
refraction changes the index carried by a transmitted ray. Every existing
surface material leaves the transmitted index unchanged — Vacuum returns 1,
and the mirrors/multilayers/filters all return rays.index_refraction — so
refractive (lens) systems weren't representable in optika.

How

Glass.index_refraction(rays) evaluates the three-term Sellmeier dispersion
equation,

n²(λ) = 1 + B₁λ²/(λ²−C₁) + B₂λ²/(λ²−C₂) + B₃λ²/(λ²−C₃),

at the ray's vacuum wavelength. Classmethods Glass.n_bk7() and Glass.f2()
return SCHOTT N-BK7 crown and F2 flint glass using published coefficients.

Tests

TestGlass runs the standard AbstractTestAbstractMaterial battery, plus
test_glass_dispersion which checks the index at the helium d Fraunhofer line
against the published value (N-BK7 → 1.5168, F2 → 1.6200), confirms normal
dispersion (higher index toward the blue), and that the glass transmits rather
than reflects.

Why

This is the prerequisite for representing refractive optics in optika, and it
gives the stop root-finding problem a real surface where n2 ≠ n1 for a
transmitted ray (relevant to the _anchor_surface discussion on #169). A
follow-up will use it for a lens tutorial once the stop-solver supports
refractive systems — see the companion issue.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.35%. Comparing base (e9a4eb5) to head (240b003).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #176   +/-   ##
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  Coverage   99.34%   99.35%           
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  Files         116      116           
  Lines        5974     6024   +50     
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+ Hits         5935     5985   +50     
  Misses         39       39           

Flag	Coverage Δ
unittests	99.35% <100.00%> (+<0.01%)	⬆️

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