Complete the UCI implementation

[kirillbobyrev]

Summary

Fourth in the sequence (after #325–#327). Fills in the remaining UCI protocol surface so the engine handles the full command/option set and emits richer analysis output.

Commands

• ponder / ponderhit. go ponder searches on the opponent's clock (unbounded); ponderhit switches to the engine's own clock via a small timer thread that stops the search once the move-time budget elapses. stop still aborts a ponder search immediately.
• register. Accepted and ignored — the engine needs no registration.

Options (advertised in the handshake, handled in setoption)

• MultiPV (spin): report the top-N root lines.
• Ponder (check): capability flag for GUIs.
• Clear Hash (button): drops the reusable search tree.

The setoption parser now handles valueless button options and boolean check options, alongside the existing spin/string ones.

info output

• seldepth (deepest simulation), hashfull (tree fill against the Hash budget), and one multipv i line per reported line.
• score mate N for forced checkmates — detected by replaying the PV to a checkmate position, so tablebase wins are not misreported as mates — otherwise score cp.

Search config (node budget + MultiPV) is bundled into mcts::Config to keep the search signature tidy.

Testing

• 143 tests pass, incl. parser tests for the new commands/options and UCI integration tests for MultiPV (multiple ranked lines), forced-mate reporting (score mate 1 / bestmove a1a8), and ponderhit.
• Manual: uci advertises all six options; MultiPV 3 emits three ranked lines with seldepth/hashfull; ponder→ponderhit returns a bestmove.
• cargo clippy --all-targets --all-features zero warnings; cargo +nightly fmt --check clean.

Next in the sequence

The transmute safety cleanup (the last of the requested items).

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