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>

SportsTimeTests/Planning/ItineraryBuilderTests.swift

@@ -303,16 +303,15 @@ struct ItineraryBuilderTests {
 
     // MARK: - Edge Case Tests
 
-    @Test("Edge: stops with nil coordinates use fallback")
-    func edge_nilCoordinates_useFallback() {
+    @Test("Edge: stops with nil coordinates are infeasible")
+    func edge_nilCoordinates_infeasible() {
         let stop1 = makeStop(city: "City1", coordinate: nil)
         let stop2 = makeStop(city: "City2", coordinate: nil)
 
         let result = ItineraryBuilder.build(stops: [stop1, stop2], constraints: constraints)
 
-        // Should use fallback distance (300 miles)
-        #expect(result != nil)
-        #expect(result?.totalDistanceMiles ?? 0 > 0)
+        // Missing coordinates = infeasible (safer to skip than show wrong drive time)
+        #expect(result == nil, "Stops with missing coordinates should be infeasible")
     }
 
     @Test("Edge: same city stops have zero distance")