Add mu-trajectory plot for the projection onto an ellipse

Cleaned-up extraction of test_mu_ellipse from proj_ellipsoid:
- quadric.py: minimal Ellipsoid class (standardization, KKT trajectory,
  extreme roots by exact bracketing, degenerate points, asymptotes).
- plot_mu_trajectories.py: the 4-panel figure, typeset with LaTeX.
- test_quadric.py: checks against brute-force sampling of the ellipse.

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

requirements.txt

@@ -0,0 +1,4 @@
+numpy
+scipy
+matplotlib>=3.6
+pytest