feat(algorithms): add information-theoretic path ranking module

Implement path ranking based on geometric mean of mutual information:
M(P) = exp((1/k) * sum(log(I(u,v)))) * exp(-lambda * k)

Key exports:
- rankPaths(): find and rank all shortest paths between nodes
- getBestPath(): convenience function for top-ranked path
- createPathRanker(): factory with pre-computed MI cache for
  efficient multiple queries on the same graph

Author: Mearman