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/Features/Trip/Views/AddItemSheet.swift

@@ -13,9 +13,7 @@ struct AddItemSheet: View {
     @Environment(\.colorScheme) private var colorScheme
 
     let tripId: UUID
-    let anchorDay: Int
-    let anchorType: CustomItineraryItem.AnchorType
-    let anchorId: String?
+    let day: Int
     let existingItem: CustomItineraryItem?
     var onSave: (CustomItineraryItem) -> Void
 
@@ -250,7 +248,7 @@ struct AddItemSheet: View {
         let item: CustomItineraryItem
 
         if let existing = existingItem {
-            // Editing existing item
+            // Editing existing item - preserve day and sortOrder
             let trimmedTitle = title.trimmingCharacters(in: .whitespaces)
             guard !trimmedTitle.isEmpty else { return }
 
@@ -259,9 +257,7 @@ struct AddItemSheet: View {
                 tripId: existing.tripId,
                 category: selectedCategory,
                 title: trimmedTitle,
-                anchorType: existing.anchorType,
-                anchorId: existing.anchorId,
-                anchorDay: existing.anchorDay,
+                day: existing.day,
                 sortOrder: existing.sortOrder,
                 createdAt: existing.createdAt,
                 modifiedAt: Date(),
@@ -274,13 +270,13 @@ struct AddItemSheet: View {
             let placeName = place.name ?? "Unknown Place"
             let coordinate = place.placemark.coordinate
 
+            // New items get sortOrder 0 (will be placed at beginning, caller can adjust)
             item = CustomItineraryItem(
                 tripId: tripId,
                 category: selectedCategory,
                 title: placeName,
-                anchorType: anchorType,
-                anchorId: anchorId,
-                anchorDay: anchorDay,
+                day: day,
+                sortOrder: 0.0,
                 latitude: coordinate.latitude,
                 longitude: coordinate.longitude,
                 address: formatAddress(for: place)
@@ -290,13 +286,13 @@ struct AddItemSheet: View {
             let trimmedTitle = title.trimmingCharacters(in: .whitespaces)
             guard !trimmedTitle.isEmpty else { return }
 
+            // New items get sortOrder 0 (will be placed at beginning, caller can adjust)
             item = CustomItineraryItem(
                 tripId: tripId,
                 category: selectedCategory,
                 title: trimmedTitle,
-                anchorType: anchorType,
-                anchorId: anchorId,
-                anchorDay: anchorDay
+                day: day,
+                sortOrder: 0.0
             )
         }
 
@@ -385,9 +381,7 @@ private struct CategoryButton: View {
 #Preview {
     AddItemSheet(
         tripId: UUID(),
-        anchorDay: 1,
-        anchorType: .startOfDay,
-        anchorId: nil,
+        day: 1,
         existingItem: nil
     ) { _ in }
 }