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/ItineraryCustomItemTests.swift

@@ -0,0 +1,153 @@
+//
+//  ItineraryCustomItemTests.swift
+//  SportsTimeTests
+//
+//  Tests for custom item movement and constraints.
+//
+
+import XCTest
+@testable import SportsTime
+
+private typealias H = ItineraryTestHelpers
+
+final class ItineraryCustomItemTests: XCTestCase {
+
+    private let testTripId = H.testTripId
+    private let testDate = H.testDate
+
+    // MARK: - Custom Item Movement Tests
+
+    func test_customItem_canMoveToAnyDay() {
+        // Given: A 5-day trip
+        let constraints = ItineraryConstraints(tripDayCount: 5, items: [])
+        let itineraryItem = ItineraryItem(tripId: testTripId, day: 1, sortOrder: 1.0, kind: .custom(CustomInfo(title: "Dinner", icon: "🍽️")))
+
+        // Custom items can go on any day
+        for day in 1...5 {
+            XCTAssertTrue(constraints.isValidPosition(for: itineraryItem, day: day, sortOrder: 50), "Custom item should be valid on Day \(day)")
+        }
+    }
+
+    func test_customItem_canMoveBeforeOrAfterGames() {
+        // Given: A day with a game at sortOrder 100
+        let game = H.makeGameItem(city: "Detroit", day: 2, sortOrder: 100)
+        let constraints = ItineraryConstraints(tripDayCount: 3, items: [game])
+
+        let customItem = ItineraryItem(tripId: testTripId, day: 2, sortOrder: 50, kind: .custom(CustomInfo(title: "Breakfast", icon: "🍳")))
+
+        // Before game (sortOrder 50) - VALID
+        XCTAssertTrue(constraints.isValidPosition(for: customItem, day: 2, sortOrder: 50), "Custom item before game should be valid")
+
+        // After game (sortOrder 150) - VALID
+        XCTAssertTrue(constraints.isValidPosition(for: customItem, day: 2, sortOrder: 150), "Custom item after game should be valid")
+    }
+
+    func test_customItem_cannotBeMovedOutsideTripRange() {
+        // Given: A 3-day trip
+        let constraints = ItineraryConstraints(tripDayCount: 3, items: [])
+        let customItem = ItineraryItem(tripId: testTripId, day: 1, sortOrder: 1.0, kind: .custom(CustomInfo(title: "Test", icon: "⭐")))
+
+        // Day 0 (before trip) - INVALID
+        XCTAssertFalse(constraints.isValidPosition(for: customItem, day: 0, sortOrder: 50), "Day 0 should be invalid")
+
+        // Day 4 (after trip) - INVALID
+        XCTAssertFalse(constraints.isValidPosition(for: customItem, day: 4, sortOrder: 50), "Day 4 should be invalid (trip is 3 days)")
+    }
+
+    // MARK: - Move Validation Tests
+
+    func test_moveValidation_customItem_blockedFromRow0() {
+        // Row 0 should always be blocked for drops
+
+        let customItem = H.makeCustomItem(day: 1, sortOrder: 1.0, title: "Moving")
+        let dayData = ItineraryDayData(id: 1, dayNumber: 1, date: testDate, games: [], items: [.customItem(customItem)], travelBefore: nil)
+
+        let controller = ItineraryTableViewController(style: .plain)
+        controller.reloadData(days: [dayData], travelValidRanges: [:], itineraryItems: [customItem])
+
+        // Attempt to move item to row 0
+        let source = IndexPath(row: 1, section: 0)
+        let proposed = IndexPath(row: 0, section: 0)
+
+        let result = controller.tableView(controller.tableView, targetIndexPathForMoveFromRowAt: source, toProposedIndexPath: proposed)
+
+        // Should NOT allow row 0
+        XCTAssertNotEqual(result.row, 0, "Row 0 should be blocked for drops")
+    }
+
+    // MARK: - Cross-Day Movement Tests
+
+    func test_moveItemBetweenDays_updatesDay() {
+        // Given: Item on Day 1, move to Day 2
+        let item = H.makeCustomItem(day: 1, sortOrder: 1.0, title: "Moving")
+
+        let day1 = ItineraryDayData(id: 1, dayNumber: 1, date: testDate, games: [], items: [.customItem(item)], travelBefore: nil)
+        let day2 = ItineraryDayData(id: 2, dayNumber: 2, date: H.dayAfter(testDate), games: [], items: [], travelBefore: nil)
+
+        var capturedDay: Int = 0
+        let controller = ItineraryTableViewController(style: .plain)
+        controller.onCustomItemMoved = { _, day, _ in
+            capturedDay = day
+        }
+        controller.reloadData(days: [day1, day2], travelValidRanges: [:], itineraryItems: [item])
+
+        // Rows: 0=Day1 header, 1=item, 2=Day2 header
+        // Move item (row 1) to after Day2 header (row 2 becomes row 2 after move)
+        controller.tableView(controller.tableView, moveRowAt: IndexPath(row: 1, section: 0), to: IndexPath(row: 2, section: 0))
+
+        XCTAssertEqual(capturedDay, 2, "Item should now belong to Day 2")
+    }
+
+    func test_moveItem_fromLastDayToFirstDay() {
+        // Given: 3-day trip with item on Day 3
+        // When: Moving to Day 1
+        // Then: Day should be 1
+
+        let item = H.makeCustomItem(day: 3, sortOrder: 1.0, title: "Moving")
+
+        let day1 = ItineraryDayData(id: 1, dayNumber: 1, date: testDate, games: [], items: [], travelBefore: nil)
+        let day2 = ItineraryDayData(id: 2, dayNumber: 2, date: H.dayAfter(testDate), games: [], items: [], travelBefore: nil)
+        let day3 = ItineraryDayData(id: 3, dayNumber: 3, date: H.dayAfter(H.dayAfter(testDate)), games: [], items: [.customItem(item)], travelBefore: nil)
+
+        var capturedDay: Int = 0
+        var capturedSortOrder: Double = 0
+        let controller = ItineraryTableViewController(style: .plain)
+        controller.onCustomItemMoved = { _, day, sortOrder in
+            capturedDay = day
+            capturedSortOrder = sortOrder
+        }
+        controller.reloadData(days: [day1, day2, day3], travelValidRanges: [:], itineraryItems: [item])
+
+        // Rows: 0=Day1 header, 1=Day2 header, 2=Day3 header, 3=item
+        // Move item to after Day1 header
+        controller.tableView(controller.tableView, moveRowAt: IndexPath(row: 3, section: 0), to: IndexPath(row: 1, section: 0))
+
+        XCTAssertEqual(capturedDay, 1, "Item should now be on Day 1")
+        XCTAssertEqual(capturedSortOrder, 1.0, accuracy: 0.01, "First item on empty day should get sortOrder 1.0")
+    }
+
+    func test_moveItem_acrossMultipleDays_withGames() {
+        // Given: Item on Day 3, games on Day 1
+        // When: Moving to Day 1 (after games)
+
+        let game1 = H.makeRichGame(city: "Detroit", hour: 14)
+        let item = H.makeCustomItem(day: 3, sortOrder: 1.0, title: "Moving")
+
+        let day1 = ItineraryDayData(id: 1, dayNumber: 1, date: testDate, games: [game1], items: [], travelBefore: nil)
+        let day2 = ItineraryDayData(id: 2, dayNumber: 2, date: H.dayAfter(testDate), games: [], items: [], travelBefore: nil)
+        let day3 = ItineraryDayData(id: 3, dayNumber: 3, date: H.dayAfter(H.dayAfter(testDate)), games: [], items: [.customItem(item)], travelBefore: nil)
+
+        var capturedDay: Int = 0
+        let controller = ItineraryTableViewController(style: .plain)
+        controller.onCustomItemMoved = { _, day, _ in
+            capturedDay = day
+        }
+        controller.reloadData(days: [day1, day2, day3], travelValidRanges: [:], itineraryItems: [item])
+
+        // Rows: 0=Day1 header, 1=games, 2=Day2 header, 3=Day3 header, 4=item
+        // Move item to row 2 (after Day1 games, before Day2 header)
+        controller.tableView(controller.tableView, moveRowAt: IndexPath(row: 4, section: 0), to: IndexPath(row: 2, section: 0))
+
+        XCTAssertEqual(capturedDay, 1, "Item moved after Day 1 games should be on Day 1")
+    }
+}