feat(09-01): add dynamic time buffers for same-day game transitions

Implements conditional time buffer logic in canTransition():
- Same-day games (doubleheaders): 2hr post-game, 0.5hr pre-game
- Multi-day games: 3hr post-game, 1hr pre-game

Enables feasible same-day combinations (LA→SD) while preventing
infeasible distant pairings (LA→SF same day).

Makes tests pass:
- plan_SameDayGamesCloseCities_BothIncluded
- plan_ThreeSameDayGames_PicksFeasibleCombinations

SportsTime/Planning/Engine/GameDAGRouter.swift

@@ -511,12 +511,7 @@ enum GameDAGRouter {
 
         let drivingHours = distanceMiles / 60.0
 
-        // Calculate available time
-        let departureTime = from.startTime.addingTimeInterval(gameEndBufferHours * 3600)
-        let deadline = to.startTime.addingTimeInterval(-3600)  // 1 hour buffer before game
-        let availableHours = deadline.timeIntervalSince(departureTime) / 3600.0
-
-        // Calculate driving days available
+        // Check if same calendar day
         let calendar = Calendar.current
         let daysBetween = calendar.dateComponents(
             [.day],
@@ -524,6 +519,27 @@ enum GameDAGRouter {
             to: calendar.startOfDay(for: to.startTime)
         ).day ?? 0
 
+        // For same-day games (doubleheaders), use shorter buffers
+        // People leave earlier and arrive closer to game time
+        let postGameBuffer: Double
+        let preGameBuffer: Double
+
+        if daysBetween == 0 {
+            // Same-day doubleheader: leave during game, arrive at game time
+            postGameBuffer = 2.0  // Leave during/right after game
+            preGameBuffer = 0.5   // Arrive closer to start time
+        } else {
+            // Different days: use standard buffers
+            postGameBuffer = gameEndBufferHours  // 3.0 hours
+            preGameBuffer = 1.0                  // 1 hour before game
+        }
+
+        // Calculate available time
+        let departureTime = from.startTime.addingTimeInterval(postGameBuffer * 3600)
+        let deadline = to.startTime.addingTimeInterval(-preGameBuffer * 3600)
+        let availableHours = deadline.timeIntervalSince(departureTime) / 3600.0
+
+        // Calculate driving hours available
         let maxDrivingHoursAvailable = daysBetween == 0
             ? max(0, availableHours)
             : Double(daysBetween) * constraints.maxDailyDrivingHours