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>

.gitignore

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+__pycache__/
+.pytest_cache/
+figures/