CLAUDE: Fix DO3 (double-3) batter advancement

Fixed incorrect batter advancement for DO3 (double-3) outcomes. The batter
was incorrectly reaching 3B instead of 2B.

DO3 means:
- DO = Double (batter reaches 2B)
- 3 = Runners advance 3 bases

Changes:
- play_resolver.py: Fixed DOUBLE_3 outcome handling (line 416)
  - Changed batter_result from 3 to 2
  - Updated description to clarify "runners advance 3 bases" not "batter to 3rd"

- play_resolver.py: Fixed X-Check DO3 handling (line 1043)
  - Changed batter_reaches from 3 to 2
  - Added comment explaining DO3 notation

Now on a DO3 + NO error:
✅ Batter reaches 2B (correct)
✅ R1 advances 3 bases → HOME (correct)
✅ R2 would advance 3 bases → HOME (correct)
✅ R3 would advance 3 bases → HOME (correct)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

backend/app/core/play_resolver.py

@@ -405,7 +405,7 @@ class PlayResolver:
             )
 
         elif outcome == PlayOutcome.DOUBLE_3:
-            # Double with extra advancement (batter to 3rd)
+            # Double with extra runner advancement (runners advance 3 bases)
             runners_advanced = self._advance_on_double_3(state)
             runs_scored = sum(1 for (from_base, to_base) in runners_advanced if to_base == 4)
 
@@ -413,9 +413,9 @@ class PlayResolver:
                 outcome=outcome,
                 outs_recorded=0,
                 runs_scored=runs_scored,
-                batter_result=3,  # Batter goes to 3rd
+                batter_result=2,  # Batter reaches 2B (it's a double)
                 runners_advanced=runners_advanced,
-                description="Double to right-center gap (batter to 3rd)",
+                description="Double to right-center gap (runners advance 3 bases)",
                 ab_roll=ab_roll,
                 is_hit=True
             )
@@ -1040,7 +1040,7 @@ class PlayResolver:
             batter_reaches = 2
         elif hit_type == 'DO3':
             base_advances = self._advance_on_double_3(state)
-            batter_reaches = 3
+            batter_reaches = 2  # DO = double (batter to 2B), 3 = runners advance 3 bases
         elif hit_type == 'TR3':
             base_advances = self._advance_on_triple(state)
             batter_reaches = 3