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/CustomItemRow.swift

@@ -78,7 +78,7 @@ struct CustomItemRow: View {
                 tripId: UUID(),
                 category: .restaurant,
                 title: "Joe's BBQ - Best brisket in Texas!",
-                anchorDay: 1
+                day: 1
             ),
             onTap: {},
             onDelete: {}
@@ -88,7 +88,7 @@ struct CustomItemRow: View {
                 tripId: UUID(),
                 category: .hotel,
                 title: "Hilton Downtown",
-                anchorDay: 1
+                day: 1
             ),
             onTap: {},
             onDelete: {}