Use quadproj instead of the local quadric module

The quadric, x(mu), f, f', mu* and the degenerate points now come from
quadproj; the script keeps small helpers for what quadproj does not
provide: mu**, the asymptotes and the ellipse boundary. The tests check
these helpers against brute-force sampling and quadproj.project.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

README.md

@@ -24,6 +24,8 @@ Text is typeset with LaTeX (`text.usetex`) when `latex` is on the `PATH`, with t
 
 ## Files
 
-- `quadric.py`: the `Ellipsoid` class (standardization, trajectory, extreme roots, degenerate points).
-- `plot_mu_trajectories.py`: the figure.
-- `test_quadric.py`: checks against brute-force sampling of the ellipse.
+- `plot_mu_trajectories.py`: the figure. The quadric, `x(mu)`, `f`, `f'`, `mu*` and the degenerate
+  points come from [quadproj](https://loicvh.gitlab.io/quadproj); the script only adds `mu**`,
+  the asymptotes and the ellipse boundary.
+- `test_plot_mu_trajectories.py`: checks these helpers against brute-force sampling of the ellipse
+  and against `quadproj.project`.