fix: implement proper bishop color detection in insufficient material check

[nikhilachale]

Summary

Fixed insufficient material draw detection to properly handle bishops on different color squares.

Problem

The previous implementation declared a draw for any two bishops regardless of their square colors. However, bishops on opposite color squares can potentially deliver checkmate in rare positions.

Solution

• Track bishop positions on the board during insufficient material detection
• Calculate square colors using (x + y) % 2 formula (0 = light, 1 = dark)
• Only declare draw when both bishops are on the same color squares
• Maintain backward compatibility with fallback to draw for edge cases

Changes

• Modified isInsufficientMaterial() method in packages/Chess/src/Game.ts
• Added bishop position tracking array
• Implemented square color calculation
• Fixed TypeScript type safety issues with non-null assertions

Testing

• All build, typecheck, and lint checks pass
• Manual verification of compiled JavaScript confirms logic is correct

Test Plan

• Build passes: pnpm build
• Typecheck passes: pnpm check-types
• Lint passes: pnpm lint
• Manual verification of bishop color logic

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