Z-score-based edge emphasis

[d2r3v]

• Add cross-team edge z-score computation based on transition probabilities (prob)
• Add minimum-support constraints (edge must appear in ≥ K teams) to avoid sparse edge explosions
• Style edges by |z| (emphasis only); do not remove edges
• Add environment var Z_EDGE_THRESHOLD and Z_MIN_TEAM_SUPPORT (defaults conservative)