Unify engine state on Game and adjudicate endgames with Syzygy

[kirillbobyrev]

Summary

First of a planned sequence connecting the game/RL layer and making stored UCI options real. Removes dead code and duplication, and makes SyzygyTablebase actually affect play.

Previously the UCI engine tracked its own position + Vec<Key> history, while game.rs held a completely dead parallel Game/Environment/RepetitionTable implementation, and the SyzygyTablebase option was parsed and stored but never used.

Changes

• Game is now the single game-state substrate. It owns the position, the repetition history, and an optional shared tablebase, and is what the engine holds (and what datagen will drive through the Environment trait next). The duplicate RepetitionTable is gone — repetition detection is a single history scan.
• Syzygy adjudication in the live search. The optional tablebase is threaded into mcts::search; leaves with few enough pieces are probed for their exact WDL value instead of using the static eval. The probe now covers all positions up to the tablebase size (the old Game::result only adjudicated at exactly max_pieces). Confirmed Tablebase<Chess> is Send + Sync, so it's shared with the background search thread via Arc.
• Search refactor to reduce boilerplate. MCTS is now a Searcher struct carrying the immutable inputs (root, history, tablebase), so simulate/expand/backup/info no longer thread them as parameters; PV construction and best-move selection share one most_visited_child helper.
• Engine wiring. Loading SyzygyTablebase now actually loads the tables (reporting the piece count), shares them with the game and each search, and state reports the loaded size. <empty> clears them.

Testing

• 131 tests pass (68 lib incl. new search-with-tablebase test; 47 chess; 9 integration incl. new tablebase-load test; 7 doctests).
• Manual: loading the test tablebases over UCI and searching a KQvK position reports a winning score and a legal move; a full self-play game runs cleanly.
• cargo clippy --all-targets --all-features reports zero warnings (the unification removed all previously-dead-code warnings); cargo +nightly fmt --check clean.

Move generation and make_move are untouched, so perft correctness is unaffected.

Next in the sequence

Transposition table + tree reuse (makes Hash real), then datagen self-play (built on this Game), a UCI-completeness pass, and the transmute safety cleanup.

🤖 Generated with Claude Code

https://claude.ai/code/session_01GVrKvkMe4xDmkRAksacBzk

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