refactor(itinerary): replace anchor-based positioning with day/sortOrder

Replace complex anchor system (anchorType, anchorId, anchorDay) with
simple (day: Int, sortOrder: Double) positioning for custom items.

Changes:
- CustomItineraryItem: Remove anchor fields, add day and sortOrder
- CKModels: Add migration fallback from old CloudKit fields
- ItineraryTableViewController: Add calculateSortOrder() for midpoint insertion
- TripDetailView: Simplify callbacks, itinerarySections, and routeWaypoints
- AddItemSheet: Take simple day parameter instead of anchor
- SavedTrip: Update LocalCustomItem SwiftData model

Benefits:
- Items freely movable via drag-and-drop
- Route waypoints follow exact visual order
- Simpler mental model: position = (day, sortOrder)
- Midpoint insertion allows unlimited reordering

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

SportsTimeTests/Domain/CustomItineraryItemTests.swift

@@ -16,15 +16,44 @@ struct CustomItineraryItemTests {
             tripId: tripId,
             category: .restaurant,
             title: "Joe's BBQ",
-            anchorDay: 1
+            day: 1
         )
 
         #expect(item.tripId == tripId)
         #expect(item.category == .restaurant)
         #expect(item.title == "Joe's BBQ")
-        #expect(item.anchorType == .startOfDay)
-        #expect(item.anchorId == nil)
-        #expect(item.anchorDay == 1)
+        #expect(item.day == 1)
+        #expect(item.sortOrder == 0.0)
+    }
+
+    @Test("Item initializes with day and sortOrder")
+    func item_InitializesWithDayAndSortOrder() {
+        let tripId = UUID()
+        let item = CustomItineraryItem(
+            tripId: tripId,
+            category: .restaurant,
+            title: "Joe's BBQ",
+            day: 1,
+            sortOrder: 1.5
+        )
+
+        #expect(item.tripId == tripId)
+        #expect(item.category == .restaurant)
+        #expect(item.title == "Joe's BBQ")
+        #expect(item.day == 1)
+        #expect(item.sortOrder == 1.5)
+    }
+
+    @Test("SortOrder defaults to 0.0")
+    func sortOrder_DefaultsToZero() {
+        let item = CustomItineraryItem(
+            tripId: UUID(),
+            category: .activity,
+            title: "City Tour",
+            day: 2
+        )
+
+        #expect(item.sortOrder == 0.0)
     }
 
     @Test("Item category has correct icons")
@@ -41,9 +70,8 @@ struct CustomItineraryItemTests {
             tripId: UUID(),
             category: .hotel,
             title: "Hilton Downtown",
-            anchorType: .afterGame,
-            anchorId: "game_123",
-            anchorDay: 2
+            day: 2,
+            sortOrder: 3.5
         )
 
         let encoded = try JSONEncoder().encode(item)
@@ -51,7 +79,7 @@ struct CustomItineraryItemTests {
 
         #expect(decoded.id == item.id)
         #expect(decoded.title == item.title)
-        #expect(decoded.anchorType == .afterGame)
-        #expect(decoded.anchorId == "game_123")
+        #expect(decoded.day == 2)
+        #expect(decoded.sortOrder == 3.5)
     }
 }