Smooth click-to-move play and accurate CPU levels

[wsams]

Click a piece to lift it, click a legal square to place it, and the moves box fills in the SAN as the piece glides. CPU strength presets now follow Stockfish 18's own Elo scale instead of the old mislabeled skill levels.

Click to move

• Click to move is on by default (toolbar, next to Flip board). Click your piece to lift it and mark legal squares, then click a square to place it.
• The moves field is written in standard notation as each move lands, including the next move number after Black.
• Escape, or a second click on the same piece, puts it back down. Promotion asks which piece to take.
• Pieces travel on a short eased arc. The board is no longer rebuilt mid-slide, which removed the hitch in the old animation.

CPU strength

Stockfish 18's UCI_Elo scale runs from 1320 to 3190 (CCRL blitz). The previous Beginner / Casual / Intermediate presets used raw Skill Level with labels around 800–1200. Those labels were wrong: Skill 0 is about 1320, and Skill 6 is about 2360, so Intermediate could outplay Master.

• Intermediate through Grandmaster now set UCI_LimitStrength and the Elo printed on the label (1320, 1400, 1600, 1800, 2100, 2500).
• Search stops at the depth where Stockfish commits the weakened move (1 + floor(skill)), with a time ceiling long enough for that ply to finish. Deeper search does not change a limited-strength move.
• Beginner and Casual stay below the 1320 floor by playing that engine and replacing half / one quarter of moves with a random legal move. Those two are marked approximate.
• Maximum is uncapped Stockfish 18 lite for up to 2 seconds.
• A new game clears the engine hash once per match, not before every move.

Dependencies

• Python: chess 1.11.2 (replaces the python-chess 1.999 alias), autopep8 2.3.2, pylint 4.1.2.
• Release tooling: semantic-release 25 and current plugins. GitHub Actions checkout and setup-node are on v7.
• chess.js 1.4.0 and the Stockfish 18 lite wasm were already current. The stockfish.js loader is updated to 18.0.8.

click_to_move_and_cpu_levels.mp4

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[github-actions]

🎉 This PR is included in version 1.8.0 🎉

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