ios: add share extension to queue and play video links

Embed an iOS and Catalyst share extension using existing discovery and player APIs. Share saved players through an App Group snapshot, preserve the queue, and verify advancing playback before reporting success.

Cover URL provider transport, settings migration, discovery addresses, duplicate handling, cancellation, and partial enqueue/play outcomes with focused tests. Document authentication and signing limits.

ios/Package.swift

@@ -0,0 +1,22 @@
+// swift-tools-version: 5.9
+import PackageDescription
+
+// A small macOS test harness for the production networking and share logic.
+// The app and extension continue to build through QueueCube.xcodeproj.
+let package = Package(
+    name: "QueueCubeShareCore",
+    platforms: [.macOS(.v14)],
+    targets: [
+        .target(
+            name: "QueueCubeShareCore",
+            path: "QueueCube/Backend",
+            exclude: ["Utilities.swift", "ServerDiscovery.swift"],
+            sources: [
+                "API.swift", "RequestBuilder.swift", "Server.swift", "Settings.swift",
+                "SharedVideo.swift", "SharePlayback.swift", "SharedInput.swift"
+            ],
+            swiftSettings: [.enableUpcomingFeature("BareSlashRegexLiterals")]
+        ),
+        .testTarget(name: "QueueCubeShareCoreTests", dependencies: ["QueueCubeShareCore"])
+    ]
+)