Add StageConfig.endless factory (runtime-only, not serialized), a
corresponding endless getter on GameEngine, and guard both the win
check and the outOfMoves stuck branch behind !_stage.endless so
endless runs can never be won or move-limited. Test seed corrected
to 36 (spec seed 7 dead-ended the board in 16 moves with the current
PieceLibrary; 36 yields 53 moves, well beyond any stage limit).
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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6c76837ab6