Modernize concurrency: Swift 6 mode, safety fixes, README rewrite

- Move package to swift-tools 6.0 with Swift 6 language mode
- Retry: saturate exponential backoff (no overflow trap), drop force
  unwrap, use Task.sleep(for:), inherit caller isolation via #isolation
- SingleFlight: propagate cancellation with waiter counting; guard
  cleanup by flight identity; remove dead AnyFlight protocol
- AsyncLifetime: drop finished tasks so they don't accumulate
- AsyncTask: lock-protect ownership and completion state
- README: correct examples (weak self, MainActor hops), Swift 6 reqs

Co-Authored-By: Claude Haiku 5.5 <noreply@anthropic.com>

Author: ANSCoder

Package.swift

@@ -1,4 +1,4 @@
-// swift-tools-version: 5.9
+// swift-tools-version: 6.0
 import PackageDescription
 
 let package = Package(
@@ -18,18 +18,13 @@ let package = Package(
     targets: [
         .target(
             name: "AsyncGuardKit",
-            path: "Sources/AsyncGuardKit",
-            swiftSettings: [
-                .enableExperimentalFeature("StrictConcurrency")
-            ]
+            path: "Sources/AsyncGuardKit"
         ),
         .testTarget(
             name: "AsyncGuardKitTests",
             dependencies: ["AsyncGuardKit"],
-            path: "Tests/AsyncGuardKitTests",
-            swiftSettings: [
-                .enableExperimentalFeature("StrictConcurrency")
-            ]
+            path: "Tests/AsyncGuardKitTests"
         )
-    ]
+    ],
+    swiftLanguageModes: [.v6]
 )