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>

README.md

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+# mu-trajectories
+
+Visualize the KKT trajectory of the Euclidean projection of a point `x0` onto an ellipse
+`x'Ax + b'x + c = 0`:
+
+    x(mu) = (I + mu A)^{-1} (x0 - mu b / 2)
+
+The figure shows the trajectory in the plane, `f(mu) = psi(x(mu))`, the distance
+`||x0 - x(mu)||` and `f'(mu)`, together with the nearest/farthest KKT points `x(mu*)`,
+`x(mu**)` and, when `x0` has a zero standardized coordinate, the degenerate point `x^d`.
+
+## Usage
+
+    pip install -r requirements.txt
+    python plot_mu_trajectories.py                 # degenerate case x0 = (0, -0.1)
+    python plot_mu_trajectories.py --x0 0.3 0.2 --output figures/generic
+    pytest
+
+The ellipse is set by `A`, `B`, `C` at the top of `plot_mu_trajectories.py`.
+
+Text is typeset with LaTeX (`text.usetex`) when `latex` is on the `PATH`, with the `cm-super` and
+`type1cm` packages; otherwise matplotlib's Computer Modern math font is used.
+
+## 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.