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/Local/SavedTrip.swift

@@ -109,9 +109,8 @@ final class LocalCustomItem {
     var tripId: UUID
     var category: String
     var title: String
-    var anchorType: String
-    var anchorId: String?
-    var anchorDay: Int
+    var day: Int
+    var sortOrder: Double
     var createdAt: Date
     var modifiedAt: Date
     var pendingSync: Bool  // True if needs to sync to CloudKit
@@ -121,9 +120,8 @@ final class LocalCustomItem {
         tripId: UUID,
         category: CustomItineraryItem.ItemCategory,
         title: String,
-        anchorType: CustomItineraryItem.AnchorType = .startOfDay,
-        anchorId: String? = nil,
-        anchorDay: Int,
+        day: Int,
+        sortOrder: Double = 0.0,
         createdAt: Date = Date(),
         modifiedAt: Date = Date(),
         pendingSync: Bool = false
@@ -132,17 +130,15 @@ final class LocalCustomItem {
         self.tripId = tripId
         self.category = category.rawValue
         self.title = title
-        self.anchorType = anchorType.rawValue
-        self.anchorId = anchorId
-        self.anchorDay = anchorDay
+        self.day = day
+        self.sortOrder = sortOrder
         self.createdAt = createdAt
         self.modifiedAt = modifiedAt
         self.pendingSync = pendingSync
     }
 
     var toItem: CustomItineraryItem? {
-        guard let category = CustomItineraryItem.ItemCategory(rawValue: category),
-              let anchorType = CustomItineraryItem.AnchorType(rawValue: anchorType)
+        guard let category = CustomItineraryItem.ItemCategory(rawValue: category)
         else { return nil }
 
         return CustomItineraryItem(
@@ -150,9 +146,8 @@ final class LocalCustomItem {
             tripId: tripId,
             category: category,
             title: title,
-            anchorType: anchorType,
-            anchorId: anchorId,
-            anchorDay: anchorDay,
+            day: day,
+            sortOrder: sortOrder,
             createdAt: createdAt,
             modifiedAt: modifiedAt
         )
@@ -164,9 +159,8 @@ final class LocalCustomItem {
             tripId: item.tripId,
             category: item.category,
             title: item.title,
-            anchorType: item.anchorType,
-            anchorId: item.anchorId,
-            anchorDay: item.anchorDay,
+            day: item.day,
+            sortOrder: item.sortOrder,
             createdAt: item.createdAt,
             modifiedAt: item.modifiedAt,
             pendingSync: pendingSync