Run tests in CI on Linux

Adds a Gitea Actions workflow running swift test in a swift:6.2 container.

The kit did not build without Apple frameworks, so the two files that need
them are wrapped in #if canImport, and the SwiftUI app target is added to
Package.swift only on macOS. Neither can exist in a Linux container.

This costs no coverage: all 43 tests are about file format and none touch
the Contacts or EventKit readers. The integration those readers represent
remains verifiable only on a Mac.

Co-Authored-By: Claude Opus 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01C5X1tYo9oxAfvoiFMh1QQr

Sources/IPodSyncKit/CalendarReader.swift

@@ -1,3 +1,6 @@
+// EventKit has no Linux equivalent. See the note in ContactsReader.swift.
+#if canImport(EventKit)
+
 import EventKit
 import Foundation
 
@@ -125,3 +128,5 @@ public actor CalendarReader {
         return "\(String(safe).prefix(64))-\(stamp)@ipod-sync"
     }
 }
+
+#endif