build: enable IsolatedDeinit feature for release builds

AppState's `isolated deinit` (SE-0371) compiles in debug but is rejected by
`swift build -c release` on the same toolchain (Swift 6.2.4). Pass the
`-enable-experimental-feature IsolatedDeinit` frontend flag so release/DMG
builds succeed.

Author: lltx

Package.swift

@@ -25,6 +25,12 @@ let package = Package(
             path: "Sources/CodeIsland",
             resources: [
                 .copy("Resources")
+            ],
+            swiftSettings: [
+                // AppState uses an `isolated deinit` (SE-0371). Release-mode builds
+                // (`swift build -c release`) reject it without this frontend flag,
+                // even though debug builds accept it on the same toolchain.
+                .enableExperimentalFeature("IsolatedDeinit")
             ]
         ),
         .executableTarget(