Fix KuhnPoker chip accounting bugs - #189

Ready to merge. Both bugs are real and correctly fixed:

1. The missing return in _init_round caused a phantom ante re-deduction after the game ended (chip sum went to −2 instead of staying zero-sum).
2. step() never moved chips on bet/call, so the pot was stuck at the antes and a called-showdown loss was indistinguishable from a fold.

I traced all four terminal lines of the action tree (check-bet-call, bet-fold, check-bet-fold, check-check): pot and per-player chips are now zero-sum and correct, and the deduction fires for the acting player before tree navigation, so resolution ordering is right. Relies on ante == bet == 1, which is the definition of Kuhn poker and hardcoded in __init__.

Verified both fixes.

1. The missing return after the end-of-game outcome meant _init_round fell through and re-shuffled/re-dealt/re-anted after the game was already decided. Adding return stops that cleanly.
2. bet/call only walked the legal-action tree and never moved chips, so betting had no effect on the final chip counts — a called bet scored the same as a check. Deducting the ante and adding it to the pot on bet/call restores real Kuhn accounting.

Deterministic check (fixed deal): chips stay zero-sum; check/check → ±1 swing, bet+call → ±2, and fold-before-call costs only the ante.

LGTM, recommend merge.