Adds module-level shannon_entropy_normalized helper and computes per-branch
action entropy from the existing monitor.action_counts data:
* ent_mag = entropy over Quarter/Half/Full distribution
* ent_dir = entropy over direction-bin distribution (indices 0-2 / 3-5 / 6-8)
Normalized to [0, 1] so 0 = full collapse, 1 = uniform.
Does not complete Task 0.10: sigma_mean (NoisyNets σ overall) remains 0.0
until Task 0.6 wires the per-branch NoisyNets σ readback (separate commit).
ec035ca3a6