Add Gomoku (five-in-a-row) game

[cweill]

Summary

Adds Gomoku as a new game in the game-agnostic pipeline. It's a placement game (no gravity): actions index board cells directly, with configurable board size and win length (default 9x9, connect 5). Registered with a single line in the central registry from #13.

Motivation

Exercises the Game interface on a different shape of game than tic-tac-toe / Connect Four — variable, larger action space (cells, not columns) — confirming the network, MCTS, and self-play stay genuinely game-agnostic. Also the first game added since the registry centralization, validating that "add a game" is now a one-line registry entry.

Implementation

• alphazero/games/gomoku.py: Gomoku(size=9, connect=5) + GomokuState, mirroring ConnectFour's line-scan win check minus gravity.
• One-line registry entry in alphazero/games/__init__.py.

Testing

• Full suite: 129 passed.
• New tests/test_gomoku.py (18 cases): all four win directions, connect-5 non-win at four marks, tiny-board (3x3) draw, canonical perspective encoding, constructor validation, registry wiring.
• Pipeline integration smoke: game_from_name("gomoku") → AlphaZeroNet → MCTS produces a valid 81-action policy.

Review focus

• Default board 9x9 / connect 5 — reasonable, or prefer a smaller default for faster self-play?
• winner() does a full-board scan each call (matches ConnectFour); fine at 9x9, could be optimized to scan only lines through the last move later.

Closes alphago-to6.