Unit tests for mobius.js

Every assertion below is a statement of rigor/network_corner_calculus.tex checked against the implementation the site actually runs: the algebra of 2×2 Möbius matrices, the parabolic compression, the Schwarzian of a C¹ piecewise-Möbius map as point masses, and the two worked examples in which hypothesis (H) fails.

These tests need JavaScript, and ES modules need the folder to be served over http (for example python3 -m http.server inside docs/). Nothing is fetched from another site.