Documents the project-wide invariant: any new distributional Q-head
(atoms, quantiles, ensembles) must expose sampleable interface and
wire into Thompson (training) + argmax (eval) action selection.
Pre-commit hook (Invariant 7) will enforce.
Architecture reflects Plan C Phase 2 amendment (commit 5de5e546a):
single-distribution C51 Thompson is the production pattern; joint
C51+IQN forward-looking under the same contract.
cb346951f4