Plot several parameter cases in one run

Replace the single A/B/C constants and --x0 option with a CASES table;
each case writes <name>.png and <name>.pdf. Add the A = diag(2, 0.75),
x0 = (0, -0.6) case.

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

README.md

@@ -12,11 +12,12 @@ The figure shows the trajectory in the plane, `f(mu) = psi(x(mu))`, the distance
 ## 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
+    python plot_mu_trajectories.py                         # every case, into figures/
+    python plot_mu_trajectories.py x_trajectories_mu_2     # a single case
     pytest
 
-The ellipse is set by `A`, `B`, `C` at the top of `plot_mu_trajectories.py`.
+Cases (`A`, `x0`, and optionally `b` and `c`) are listed in `CASES` at the top of
+`plot_mu_trajectories.py`; each one produces `<name>.png` and `<name>.pdf`.
 
 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.