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>

.gitignore

@@ -1,3 +1,4 @@
 __pycache__/
 .pytest_cache/
 figures/
+venv/