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>

SportsTime/Core/Models/Domain/CustomItineraryItem.swift

@@ -11,10 +11,8 @@ struct CustomItineraryItem: Identifiable, Codable, Hashable {
     let tripId: UUID
     var category: ItemCategory
     var title: String
-    var anchorType: AnchorType
-    var anchorId: String?
-    var anchorDay: Int
-    var sortOrder: Int  // For ordering within same anchor position
+    var day: Int           // Day number (1-indexed)
+    var sortOrder: Double  // Position within day (allows insertion between items)
     let createdAt: Date
     var modifiedAt: Date
 
@@ -39,10 +37,8 @@ struct CustomItineraryItem: Identifiable, Codable, Hashable {
         tripId: UUID,
         category: ItemCategory,
         title: String,
-        anchorType: AnchorType = .startOfDay,
-        anchorId: String? = nil,
-        anchorDay: Int,
-        sortOrder: Int = 0,
+        day: Int,
+        sortOrder: Double = 0.0,
         createdAt: Date = Date(),
         modifiedAt: Date = Date(),
         latitude: Double? = nil,
@@ -53,9 +49,7 @@ struct CustomItineraryItem: Identifiable, Codable, Hashable {
         self.tripId = tripId
         self.category = category
         self.title = title
-        self.anchorType = anchorType
-        self.anchorId = anchorId
-        self.anchorDay = anchorDay
+        self.day = day
         self.sortOrder = sortOrder
         self.createdAt = createdAt
         self.modifiedAt = modifiedAt
@@ -97,10 +91,4 @@ struct CustomItineraryItem: Identifiable, Codable, Hashable {
             }
         }
     }
-
-    enum AnchorType: String, Codable {
-        case startOfDay
-        case afterGame
-        case afterTravel
-    }
 }