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>

requirements.txt

@@ -1,4 +1,6 @@
 numpy
 scipy
 matplotlib>=3.6
+quadproj>=2.1
+imageio  # imported by quadproj but not declared as a dependency
 pytest