Added support for using the object surface as the field stop of a SequentialSystem

[jacobdparker]

Second PR in the stop-solver stack — stacked on #168 (the diff shown here is relative to that branch).

Problem

Marking the object surface (with an angular, dimensionless aperture) as the field stop is the natural configuration for dispersive systems — at a single wavelength of a dispersed spectrum, most of the sensor outline is unreachable, so targeting the sensor wire is ill-posed. But this configuration was broken in three independent ways:

1. The position-variable branch of _calc_rayfunction_stops_only called sag() with a 2-component vector → TypeError before the solve started.
2. The solved outputs were never broadcast over both stop axes → axis error in the field_min/max / pupil_min/max reductions.
3. The hardcoded initial guess (rays at the first stop's origin, direction +z) produces NaN first residuals whenever the next surface is far off that axis (e.g. the feed mirror of a Rowland-circle spectrograph, 174 mm off-axis in FURST) or on steep grazing flanks — and Newton cannot recover from NaN.

Changes

• zfunc builds a proper 3-D vector before calling sag().
• _calc_rayfunction_stops broadcasts position and direction against each other before the stop-axis reductions.
• New _anchor_surface helper: the seed now aims each ray at its own target point on the last stop surface when no powered surface lies between the stops (nearly exact — the grazing solve converges in ~5 iterations), and otherwise at the center of the first powered surface.

Tests

Adds TestSequentialSystemGrazingSpectrograph: a grazing-incidence paraboloid + transmission-grating spectrograph whose source is the field stop, asserting the recovered field equals the source's angular radius (0.25°) to 1e-6 deg. Also verified against the FURST Spectrograph model from Kankelborg-Group/furst-optics#25, which recovers field_max = ±0.26656° (the sunpy solar angular radius).

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.50%. Comparing base (901c539) to head (74c747b).

Additional details and impacted files

@@                    Coverage Diff                    @@
##           fix/solver-robustness     #169      +/-   ##
=========================================================
+ Coverage                  99.34%   99.50%   +0.15%     
=========================================================
  Files                        116      116              
  Lines                       5974     6042      +68     
=========================================================
+ Hits                        5935     6012      +77     
+ Misses                        39       30       -9     

Flag	Coverage Δ
unittests	99.50% <100.00%> (+0.15%)	⬆️

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[roytsmart]

It's not enough for there to be a sag, the material must also have a different index of refraction