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/CloudKit/CKModels.swift

@@ -632,16 +632,19 @@ struct CKCustomItineraryItem {
     static let tripIdKey = "tripId"
     static let categoryKey = "category"
     static let titleKey = "title"
-    static let anchorTypeKey = "anchorType"
-    static let anchorIdKey = "anchorId"
-    static let anchorDayKey = "anchorDay"
-    static let sortOrderKey = "sortOrder"
+    static let dayKey = "day"                    // NEW: replaces anchorDay
+    static let sortOrderDoubleKey = "sortOrderDouble"  // NEW: Double instead of Int
     static let createdAtKey = "createdAt"
     static let modifiedAtKey = "modifiedAt"
     // Location fields for mappable items
     static let latitudeKey = "latitude"
     static let longitudeKey = "longitude"
     static let addressKey = "address"
+    // DEPRECATED - kept for migration reads only
+    static let anchorTypeKey = "anchorType"
+    static let anchorIdKey = "anchorId"
+    static let anchorDayKey = "anchorDay"
+    static let sortOrderKey = "sortOrder"
 
     let record: CKRecord
 
@@ -658,10 +661,8 @@ struct CKCustomItineraryItem {
         record[CKCustomItineraryItem.tripIdKey] = item.tripId.uuidString
         record[CKCustomItineraryItem.categoryKey] = item.category.rawValue
         record[CKCustomItineraryItem.titleKey] = item.title
-        record[CKCustomItineraryItem.anchorTypeKey] = item.anchorType.rawValue
-        record[CKCustomItineraryItem.anchorIdKey] = item.anchorId
-        record[CKCustomItineraryItem.anchorDayKey] = item.anchorDay
-        record[CKCustomItineraryItem.sortOrderKey] = item.sortOrder
+        record[CKCustomItineraryItem.dayKey] = item.day
+        record[CKCustomItineraryItem.sortOrderDoubleKey] = item.sortOrder
         record[CKCustomItineraryItem.createdAtKey] = item.createdAt
         record[CKCustomItineraryItem.modifiedAtKey] = item.modifiedAt
         // Location fields (nil values are not stored in CloudKit)
@@ -679,15 +680,30 @@ struct CKCustomItineraryItem {
               let categoryString = record[CKCustomItineraryItem.categoryKey] as? String,
               let category = CustomItineraryItem.ItemCategory(rawValue: categoryString),
               let title = record[CKCustomItineraryItem.titleKey] as? String,
-              let anchorTypeString = record[CKCustomItineraryItem.anchorTypeKey] as? String,
-              let anchorType = CustomItineraryItem.AnchorType(rawValue: anchorTypeString),
-              let anchorDay = record[CKCustomItineraryItem.anchorDayKey] as? Int,
               let createdAt = record[CKCustomItineraryItem.createdAtKey] as? Date,
               let modifiedAt = record[CKCustomItineraryItem.modifiedAtKey] as? Date
         else { return nil }
 
-        let anchorId = record[CKCustomItineraryItem.anchorIdKey] as? String
-        let sortOrder = record[CKCustomItineraryItem.sortOrderKey] as? Int ?? 0
+        // Read new fields, with migration fallback from old fields
+        let day: Int
+        if let newDay = record[CKCustomItineraryItem.dayKey] as? Int {
+            day = newDay
+        } else if let oldDay = record[CKCustomItineraryItem.anchorDayKey] as? Int {
+            // Migration: use old anchorDay
+            day = oldDay
+        } else {
+            return nil
+        }
+
+        let sortOrder: Double
+        if let newSortOrder = record[CKCustomItineraryItem.sortOrderDoubleKey] as? Double {
+            sortOrder = newSortOrder
+        } else if let oldSortOrder = record[CKCustomItineraryItem.sortOrderKey] as? Int {
+            // Migration: convert old Int sortOrder to Double
+            sortOrder = Double(oldSortOrder)
+        } else {
+            sortOrder = 0.0
+        }
 
         // Location fields (optional - nil if not stored)
         let latitude = record[CKCustomItineraryItem.latitudeKey] as? Double
@@ -699,9 +715,7 @@ struct CKCustomItineraryItem {
             tripId: tripId,
             category: category,
             title: title,
-            anchorType: anchorType,
-            anchorId: anchorId,
-            anchorDay: anchorDay,
+            day: day,
             sortOrder: sortOrder,
             createdAt: createdAt,
             modifiedAt: modifiedAt,