Add Sun Country (SY) load integration

Sun Country runs Navitaire (same PSS as JSX) but exposes their public
availability search endpoint that returns BETTER load data than AA:
per-flight `capacity` AND `sold` (booked passenger count), so we can
compute exact load factor.

Implementation:
- AirlineLoadService.fetchSunCountryLoad: POSTs to
  syprod-api.suncountry.com/api/nsk/v4/availability/search/simple.
  Parses results→trips→journeysAvailableByMarket, matches by flight
  number, pulls capacity + sold + equipmentType from legInfo.
- Returns a single Economy CabinLoad with capacity/booked = sold.
  No standby program — SY is single-cabin Y.
- Auth: Azure APIM subscription key + a long-lived dotREZ JWT
  (both static, captured from suncountry.com network traffic, neither
  is a user session token).
- Anti-bot: Imperva WAF in front of syprod-api.suncountry.com is gated
  on User-Agent + Referer + Origin headers. applySunCountryBrowserHeaders
  mirrors the pattern we use for UA / AA. NO WebView needed.
- Explicit ⚠️ log when 403 Incapsula response detected, pointing at
  the header helper.

Test infrastructure:
- knownDailyFlights now carries a dayOffset (today vs tomorrow) per
  carrier — different upstreams have different snapshot windows:
  AM is T-1d..T+0 (today); SY's Navitaire only returns future flights
  (tomorrow); others default to tomorrow as a safer choice.
- Added test_SY_sunCountry with hubs MSP/LAS/MCO/DEN. Fallback is
  SY104 LAS-MSP tomorrow.

Docs:
- AIRLINE_INTEGRATION_GUIDE: SY status row + full section 5c covering
  endpoint, auth, headers, response shape, failure modes, and how to
  re-capture tokens when they rotate.

Reverse-engineering notes:
- SY app is Flutter (Dart AOT) — bridge smali is minimal. Strings
  extracted from libapp.so revealed isNonRevTrip/isStandby/
  inventoryControl keywords + the syprod-api hostname.
- Token endpoint is PUT (not POST). Returns {"data":null} — token is
  the existing Authorization JWT, not a session refresh.
- Confirmed working from plain curl with browser headers (no Imperva
  TLS-fingerprint gate beyond UA/Referer/Origin).

Test run 2026-05-26 (xcodebuild test):
  ✅ AA, AM, AS, B6, EK, KE, SY (capacity=186 sold=184 load=99%), UA
  ⏭️ XE
  8 passing, 1 skipped, 0 failures, 11s total.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

FlightsTests/AirlineLoadIntegrationTests.swift

@@ -45,10 +45,16 @@ final class AirlineLoadIntegrationTests: XCTestCase {
     /// route-explorer's schedule feed). Each entry is a well-known daily
     /// operation that's been stable over time; if any of these stop
     /// operating, update the entry.
-    private static let knownDailyFlights: [String: (flightNumber: String, origin: String, destination: String)] = [
-        "EK": ("201", "JFK", "DXB"),   // Emirates JFK → Dubai, daily flagship
-        "KE": ("82",  "JFK", "ICN"),   // Korean Air JFK → Incheon, daily
-        "AM": ("58",  "MEX", "MTY"),   // Aeromexico MEX → Monterrey, multiple daily
+    ///
+    /// `dayOffset` controls which day's flight to probe:
+    /// - `0` (today) for carriers whose snapshot window is T-1d to T+0 (AM)
+    /// - `1` (tomorrow) for carriers whose API only returns future flights
+    ///   (SY's Navitaire availability/search drops already-departed legs)
+    private static let knownDailyFlights: [String: (flightNumber: String, origin: String, destination: String, dayOffset: Int)] = [
+        "EK": ("201", "JFK", "DXB", 1),   // Emirates JFK → Dubai, daily flagship
+        "KE": ("82",  "JFK", "ICN", 1),   // Korean Air JFK → Incheon, daily
+        "AM": ("58",  "MEX", "MTY", 0),   // Aeromexico — snapshot only T-1d/T+0
+        "SY": ("104", "LAS", "MSP", 1),   // Sun Country — Navitaire shows future only
     ]
 
     // MARK: - Per-airline tests
@@ -104,6 +110,16 @@ final class AirlineLoadIntegrationTests: XCTestCase {
         )
     }
 
+    func test_SY_sunCountry() async throws {
+        // Sun Country runs on Navitaire; the load endpoint takes flight
+        // number + route + date. Route-explorer doesn't cover SY well, so
+        // most runs hit the known-daily fallback (SY104 LAS-MSP).
+        try await runAirlineLoadTest(
+            carrier: "SY",
+            hubs: ["MSP", "LAS", "MCO", "DEN"]
+        )
+    }
+
     func test_XE_jsx() async throws {
         // JSX uses a WKWebView path that needs a host scene / main thread.
         // Skipped here; manual verification via the app remains.
@@ -159,12 +175,15 @@ final class AirlineLoadIntegrationTests: XCTestCase {
         // Fallback: known-good daily flight. Triggers when route-explorer
         // found nothing OR when the discovered flight returned nil (e.g. a
         // regional carrier op that isn't in the upstream load system).
+        // dayOffset in the table controls today-vs-tomorrow based on each
+        // carrier's snapshot window quirks.
         if let known = Self.knownDailyFlights[carrier] {
-            NSLog("[\(carrier)Test] Using known daily \(carrier)\(known.flightNumber) \(known.origin)→\(known.destination)")
+            let probeDate = Date().addingTimeInterval(TimeInterval(known.dayOffset * 86400))
+            NSLog("[\(carrier)Test] Using known daily \(carrier)\(known.flightNumber) \(known.origin)→\(known.destination) +\(known.dayOffset)d")
             let load = await loadService.fetchLoad(
                 airlineCode: carrier,
                 flightNumber: known.flightNumber,
-                date: Date(),
+                date: probeDate,
                 origin: known.origin,
                 destination: known.destination,
                 departureTime: nil