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/ItineraryEdgeCaseTests.swift

@@ -0,0 +1,67 @@
+//
+//  ItineraryEdgeCaseTests.swift
+//  SportsTimeTests
+//
+//  Tests for edge cases in itinerary display and manipulation.
+//
+
+import XCTest
+@testable import SportsTime
+
+private typealias H = ItineraryTestHelpers
+
+final class ItineraryEdgeCaseTests: XCTestCase {
+
+    private let testDate = H.testDate
+
+    // MARK: - Empty/Minimal Day Tests
+
+    func test_emptyDay_onlyShowsHeader() {
+        // Given: A day with no games and no items
+        let dayData = ItineraryDayData(id: 1, dayNumber: 1, date: testDate, games: [], items: [], travelBefore: nil)
+
+        let controller = ItineraryTableViewController(style: .plain)
+        controller.reloadData(days: [dayData], travelValidRanges: [:])
+
+        let rowCount = controller.tableView(controller.tableView, numberOfRowsInSection: 0)
+        XCTAssertEqual(rowCount, 1, "Empty day should only have header row")
+    }
+
+    func test_restDay_withTravelBefore_onlyShowsHeader() {
+        // Given: A rest day with travelBefore set (legacy field)
+        // Semantic model: travelBefore is IGNORED - travel must be in items to appear
+        let travel = H.makeTravelSegment(from: "Chicago", to: "Detroit")
+        let dayData = ItineraryDayData(id: 1, dayNumber: 1, date: testDate, games: [], items: [], travelBefore: travel)
+
+        let controller = ItineraryTableViewController(style: .plain)
+        controller.reloadData(days: [dayData], travelValidRanges: [:])
+
+        let rowCount = controller.tableView(controller.tableView, numberOfRowsInSection: 0)
+        XCTAssertEqual(rowCount, 1, "travelBefore is ignored - only header should appear")
+    }
+
+    func test_singleGameDay_showsHeaderAndGame() {
+        // Given: A day with one game
+        let games = [H.makeRichGame(city: "Detroit", hour: 19)]
+        let dayData = ItineraryDayData(id: 1, dayNumber: 1, date: testDate, games: games, items: [], travelBefore: nil)
+
+        let controller = ItineraryTableViewController(style: .plain)
+        controller.reloadData(days: [dayData], travelValidRanges: [:])
+
+        let rowCount = controller.tableView(controller.tableView, numberOfRowsInSection: 0)
+        XCTAssertEqual(rowCount, 2, "Day with one game should have 2 rows: header + games")
+    }
+
+    // MARK: - Multi-Day Trip Tests
+
+    func test_multiDayTrip_allDaysRepresented() {
+        // Given: A 5-day trip
+        let days = H.makeDays(count: 5)
+
+        let controller = ItineraryTableViewController(style: .plain)
+        controller.reloadData(days: days, travelValidRanges: [:])
+
+        let rowCount = controller.tableView(controller.tableView, numberOfRowsInSection: 0)
+        XCTAssertEqual(rowCount, 5, "5-day trip with empty days should have 5 header rows")
+    }
+}