Harden UCI/FEN parsing, fix engine bugs, upgrade toolchain and dependencies

[kirillbobyrev]

Summary

A codebase review pass: bug fixes in UCI/FEN handling and game-state tracking, plus a toolchain/dependency upgrade.

Bug fixes (fix: commit)

• UCI go time controls were parsed as microseconds — the UCI protocol specifies milliseconds, so the engine would have been given 1000× less time than intended once time management lands.
• Panics on untrusted UCI input, replaced with graceful errors:
  • non-ASCII move strings panicked on byte slicing (&uci[..2] on a multi-byte char boundary);
  • unknown promotion characters hit unreachable!();
  • e2e2-style moves tripped a debug assertion in Move::new;
  • setoption name Hash value (trailing value with no argument) panicked with an out-of-bounds index.
• go argument scanning skipped two tokens after any unrecognized keyword, so valueless tokens (ponder, unknown extensions) swallowed the keyword that followed them (go ponder wtime 1000 lost wtime).
• Unsound transmute in Move::promotion() — transmuting a raw u8 to Option<Promotion> relies on an unspecified niche layout; replaced with an explicit match.
• FEN validation now rejects positions where the side not to move is in check (including adjacent kings), closing the "probe opposite checks" TODO. Such positions break move-generator invariants downstream.
• Halfmove clock overflow: u8 incremented without bound panicked in debug builds past 255 plies without capture/pawn move; now saturates.
• RepetitionTable::record returned false for the 4th+ occurrence of a position (== 3 → >= 3), which could un-flag a threefold draw.
• Engine loop robustness: input read errors and invalid moves in position commands now propagate as errors instead of panic!/unreachable!().
• Fuzz target fix: generate_moves called is_check() on a Result and did not compile.

Upgrades (chore: commit)

• MSRV → 1.95 (required by shakmaty 0.30); the build workflow's MSRV job was pinned to 1.80.0, which cannot build the 2024 edition, and now matches rust-version.
• itertools 0.13→0.15, rand 0.9→0.10 (+ rand_distr 0.5→0.6; small_rng is built-in now and random() moved to RngExt), shadow-rs 1→2, shakmaty 0.27→0.30, shakmaty-syzygy 0.25→0.28, indicatif 0.17→0.18, criterion 0.5→0.8, plus a full cargo update. Fuzz crate deps aligned.
• Fixed the new clippy::useless_borrows_in_formatting perf lint (denied via clippy::perf) surfaced by clippy 1.97.

Testing

• cargo test: 102 tests + doctests pass.
• cargo test --release -- --ignored: all 12 expensive perft / differential tests against shakmaty pass (move generation exactness verified after the movegen-adjacent changes).
• cargo clippy --all-targets clean (only pre-existing dead-code warnings in search stubs).
• New regression tests: malformed UCI moves, setoption/go parsing edge cases, opposite-check and adjacent-king FEN rejection.

🤖 Generated with Claude Code

https://claude.ai/code/session_01GVrKvkMe4xDmkRAksacBzk

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