Commute time distance

TODO. Will cover:

• Expected hitting + return time under the harmonic random walk.
• Doyle–Snell identity: $h(u, v) + h(v, u) = \left(\sum_{x,y} C_{x,y}\right) \cdot d_{R}(u, v)$.
• IsHypergraphMetric instance status (currently a 5-hypothesis stub).