Add implementation code for all 4 improvement plan phases

Production changes:
- TravelEstimator: remove 300mi fallback, return nil on missing coords
- TripPlanningEngine: add warnings array, empty sports warning, inverted
  date range rejection, must-stop filter, segment validation gate
- GameDAGRouter: add routePreference parameter with preference-aware
  bucket ordering and sorting in selectDiverseRoutes()
- ScenarioA-E: pass routePreference through to GameDAGRouter
- ScenarioA: track games with missing stadium data
- ScenarioE: add region filtering for home games
- TravelSegment: add requiresOvernightStop and travelDays() helpers

Test changes:
- GameDAGRouterTests: +252 lines for route preference verification
- TripPlanningEngineTests: +153 lines for segment validation, date range,
  empty sports
- ScenarioEPlannerTests: +119 lines for region filter tests
- TravelEstimatorTests: remove obsolete fallback distance tests
- ItineraryBuilderTests: update nil-coords test expectation

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

SportsTime/Core/Models/Domain/TravelSegment.swift

@@ -58,6 +58,17 @@ struct TravelSegment: Identifiable, Codable, Hashable {
     var estimatedDrivingHours: Double { durationHours }
     var estimatedDistanceMiles: Double { distanceMiles }
 
+    /// Whether this segment requires an overnight stop based on driving time.
+    /// Segments over 8 hours of driving require rest.
+    var requiresOvernightStop: Bool {
+        durationHours > 8.0
+    }
+
+    /// Number of travel days this segment spans (accounting for daily driving limits).
+    func travelDays(maxDailyHours: Double = 8.0) -> Int {
+        max(1, Int(ceil(durationHours / maxDailyHours)))
+    }
+
     var formattedDistance: String {
         String(format: "%.0f mi", distanceMiles)
     }