test(update-checker): rewrite tests to exercise real production code

REPLACED: three worthless tests that only tested test code, not production:
- statusMessageUpToDateFormat built the expected string locally and matched it
- statusMessageUpdateReadyFormat same self-referential test
- statusChannelsAreIndependent was literally #expect(true, ...)

ADDED: four real tests that drive production code:
- dubaiTimeCheckTimeFormat: assert DubaiTime.checkTime() formats as HH:MM Dubai
- manualCheckUpToDateIncludesTimestamp: inject stub appcast via testAppcastJSON seam,
  drive UpdateChecker.checkNow(manual: true), verify statusMessage matches exact format
- automaticCheckUpToDateLeavesMessageNil: verify automatic check (manual: false) leaves
  statusMessage nil when up-to-date
- statusChannelsAreIndependent: construct real AppModel via AppDelegate.makeLaunchModel(),
  assert setStatus() does NOT affect updateStatusMessage, setUpdateStatus() does NOT
  affect statusLine, and vice versa. PROVES the defect is caught: test fails with 3 issues
  if updateStatusMessage is reverted to an alias of statusLine.

ADDED: testAppcastJSON seam to UpdateChecker for test injection of appcast data.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012ZiTXPbPCSjzPVsfoweAbp

Sources/Shotdeck/UpdateChecker.swift

@@ -36,6 +36,8 @@ final class UpdateChecker {
     /// this value (including nil) is returned instead of checking the file system.
     var snapshotPreviousVersionOverride: String?
     var snapshotUsesPreviousVersionOverride: Bool = false
+    /// Test seam: override appcast JSON. When set, returns this instead of fetching from URL.
+    var testAppcastJSON: String?
 
     init() {
         let config = URLSessionConfiguration.ephemeral
@@ -359,7 +361,12 @@ final class UpdateChecker {
     }
 
     private func fetchAppcast() async throws -> Appcast {
-        let data = try await fetchData(from: Self.resolvedAppcastURL())
+        let data: Data
+        if let testJSON = testAppcastJSON {
+            data = testJSON.data(using: .utf8) ?? Data()
+        } else {
+            data = try await fetchData(from: Self.resolvedAppcastURL())
+        }
         return try JSONDecoder().decode(Appcast.self, from: data)
     }