refactor(itinerary): extract reordering logic into pure functions

Extract all itinerary reordering logic from ItineraryTableViewController into
ItineraryReorderingLogic.swift for testability. Key changes:

- Add flattenDays, dayNumber, travelRow, simulateMove pure functions
- Add calculateSortOrder with proper region classification (before/after games)
- Add computeValidDestinationRowsProposed with simulation+validation pattern
- Add coordinate space conversion helpers (proposedToOriginal, originalToProposed)
- Fix DragZones coordinate space mismatch (was mixing proposed/original indices)
- Add comprehensive documentation of coordinate space conventions

Test coverage includes:
- Row flattening order and semantic travel model
- Sort order calculation for before/after games regions
- Travel constraints validation
- DragZones coordinate space correctness
- Coordinate conversion helpers
- Edge cases (empty days, multi-day trips)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

SportsTimeTests/Features/Trip/ItineraryTestHelpers.swift

@@ -0,0 +1,125 @@
+//
+//  ItineraryTestHelpers.swift
+//  SportsTimeTests
+//
+//  Shared test fixtures and helpers for Itinerary tests.
+//
+
+import Foundation
+@testable import SportsTime
+
+/// Shared test fixtures for itinerary tests
+enum ItineraryTestHelpers {
+    static let testTripId = UUID()
+    static let testDate = Date()
+
+    // MARK: - Day Helpers
+
+    static func makeDays(count: Int, from baseDate: Date = testDate) -> [ItineraryDayData] {
+        return (0..<count).map { i in
+            ItineraryDayData(
+                id: i + 1,
+                dayNumber: i + 1,
+                date: Calendar.current.date(byAdding: .day, value: i, to: baseDate)!,
+                games: [],
+                items: [],
+                travelBefore: nil
+            )
+        }
+    }
+
+    static func dayAfter(_ date: Date) -> Date {
+        Calendar.current.date(byAdding: .day, value: 1, to: date)!
+    }
+
+    // MARK: - Travel Helpers
+
+    static func makeTravelSegment(from: String, to: String) -> TravelSegment {
+        TravelSegment(
+            fromLocation: LocationInput(name: from, coordinate: nil),
+            toLocation: LocationInput(name: to, coordinate: nil),
+            travelMode: .drive,
+            distanceMeters: 500_000,
+            durationSeconds: 18000
+        )
+    }
+
+    static func makeTravelItem(from: String, to: String, day: Int, sortOrder: Double) -> ItineraryItem {
+        ItineraryItem(
+            tripId: testTripId,
+            day: day,
+            sortOrder: sortOrder,
+            kind: .travel(TravelInfo(fromCity: from, toCity: to))
+        )
+    }
+
+    // MARK: - Game Helpers
+
+    static func makeRichGame(city: String, hour: Int, baseDate: Date = testDate) -> RichGame {
+        var dateComponents = Calendar.current.dateComponents([.year, .month, .day], from: baseDate)
+        dateComponents.hour = hour
+        let gameTime = Calendar.current.date(from: dateComponents)!
+
+        let game = Game(
+            id: "game-\(city)-\(UUID().uuidString.prefix(4))",
+            homeTeamId: "team-\(city)",
+            awayTeamId: "team-visitor",
+            stadiumId: "stadium-\(city)",
+            dateTime: gameTime,
+            sport: .mlb,
+            season: "2026",
+            isPlayoff: false
+        )
+
+        let stadium = Stadium(
+            id: "stadium-\(city)",
+            name: "\(city) Stadium",
+            city: city,
+            state: "XX",
+            latitude: 40.0,
+            longitude: -80.0,
+            capacity: 40000,
+            sport: .mlb
+        )
+
+        let homeTeam = Team(
+            id: "team-\(city)",
+            name: "\(city) Team",
+            abbreviation: String(city.prefix(3)).uppercased(),
+            sport: .mlb,
+            city: city,
+            stadiumId: "stadium-\(city)"
+        )
+
+        let awayTeam = Team(
+            id: "team-visitor",
+            name: "Visitor Team",
+            abbreviation: "VIS",
+            sport: .mlb,
+            city: "Visiting",
+            stadiumId: "stadium-visitor"
+        )
+
+        return RichGame(game: game, homeTeam: homeTeam, awayTeam: awayTeam, stadium: stadium)
+    }
+
+    static func makeGameItem(city: String, day: Int, sortOrder: Double = 100) -> ItineraryItem {
+        ItineraryItem(
+            tripId: testTripId,
+            day: day,
+            sortOrder: sortOrder,
+            kind: .game(gameId: "game-\(city)-\(UUID().uuidString.prefix(4))")
+        )
+    }
+
+    // MARK: - Custom Item Helpers
+
+    static func makeCustomItem(day: Int, sortOrder: Double, title: String) -> ItineraryItem {
+        ItineraryItem(
+            tripId: testTripId,
+            day: day,
+            sortOrder: sortOrder,
+            kind: .custom(CustomInfo(title: title, icon: "🍽️"))
+        )
+    }
+}