[8/9] Add futility pruning

Implements futility pruning to skip quiet moves that can't improve alpha:

**Futility Pruning:**
- At low depths (1-2), compute static evaluation
- If eval + margin < alpha, quiet moves can't help
- Skip quiet moves (no capture, check, or promotion)
- Never prune the first move (might be the only good one)

**Margin Calculation:**
- Depth 1: 100 centipawns margin
- Depth 2: 200 centipawns margin
- Larger margin at deeper depths allows for more potential improvement

**Conditions for pruning:**
- Depth <= 2
- Not in check (check positions are critical)
- Static eval + margin < alpha
- Move is quiet (not capture/check/promotion)
- Not the first move in the list

This is a forward pruning technique that can miss some moves, but
the marginsare conservative enough to rarely affect results while
significantly reducing nodes searched.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: luccabb

moonfish/engines/alpha_beta.py

@@ -302,11 +302,32 @@ def negamax(
 
         in_check = board.is_check()
 
+        # Futility pruning: if static eval is far below alpha, quiet moves won't help
+        # Only compute if we might use it (low depth, not in check)
+        futility_margin = 0
+        can_futility_prune = False
+        if depth <= 2 and not in_check:
+            static_eval = self.eval_board(board)
+            # Margin increases with depth: depth 1 = 100cp, depth 2 = 200cp
+            futility_margin = 100 * depth
+            can_futility_prune = static_eval + futility_margin < alpha
+
         for move_index, move in enumerate(moves):
             is_capture = board.is_capture(move)
             gives_check = board.gives_check(move)
             is_promotion = move.promotion is not None
 
+            # Futility pruning: skip quiet moves that can't raise alpha
+            # Conditions: low depth, not PV move, quiet move, not in check
+            if (
+                can_futility_prune
+                and move_index > 0  # Don't prune first move
+                and not is_capture
+                and not gives_check
+                and not is_promotion
+            ):
+                continue
+
             # make the move
             board.push(move)