The isStalemate() called the Stockfish API on every move just to check
if the opponent had legal moves — doubling API calls from 80 to 160 per
game. Now relies on the API returning no best_move as game-over signal.
Differentiates checkmate vs stalemate by eval magnitude.

Also reduced max moves from 80 to 60 — bot games at these levels
should resolve well before that.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>