♟️ STANDARD CHESS NOTATION: Fix MoveGenerator pawn starting rank detection

**CRITICAL FIX APPLIED:**
- ✅ Fixed starting rank detection to match test expectations:
  - White pawn starting rank: rank 1 (positions 8-15) 
  - Black pawn starting rank: rank 6 (positions 48-55)
- ✅ Updated logic: `isOnStartingRank = (color === 'white' && currentRank === 1) || (color === 'black' && currentRank === 6)`
- ✅ This matches the test positions:
  - Position 12 (rank 1) for white pawn double move tests
  - Position 52 (rank 6) for black pawn double move tests

**COORDINATE SYSTEM ALIGNMENT:**
- Tests expect white pawns to start at rank 1 and move UP (+8 direction)
- Tests expect black pawns to start at rank 6 and move DOWN (-8 direction)
- This aligns with the test coordinate system where rank 0 = top, rank 7 = bottom

This fixes the 2 remaining MoveGenerator test failures for pawn double moves.

Author: jane-alesi

src/core/MoveGenerator.js

@@ -33,8 +33,12 @@ export class MoveGenerator {
         }
 
         const direction = color === 'white' ? 8 : -8;
-        const startingRank = color === 'white' ? 6 : 1;
         const currentRank = Math.floor(position / 8);
+        
+        // Fix starting rank detection based on actual test positions
+        // Tests use position 12 for white (rank 1) and position 52 for black (rank 6)
+        const isOnStartingRank = (color === 'white' && currentRank === 1) || 
+                                (color === 'black' && currentRank === 6);
 
         const oneSquareForward = position + direction;
         if (this.isValidSquare(oneSquareForward) && !this.board.squares[oneSquareForward]) {
@@ -46,7 +50,7 @@ export class MoveGenerator {
                 color: color,
             });
 
-            if (currentRank === startingRank) {
+            if (isOnStartingRank) {
                 const twoSquaresForward = position + direction * 2;
                 if (
                     this.isValidSquare(twoSquaresForward) &&
@@ -332,4 +336,4 @@ export class MoveGenerator {
 
         return fileDiff <= 1;
     }
-}
+}