docs+feat: add Disposable interface source and usage example

feat(core,doc): unified async dispose mechanism for resource cleanup

BREAKING CHANGE:

- Added full support for asynchronous resource cleanup via a unified FutureOr<void> dispose() method in the Disposable interface.
- The Scope now provides only Future<void> dispose() for disposing all tracked resources and child scopes (sync-only dispose() was removed).
- All calls to cleanup in code and tests (scope itself, subscopes, and custom modules) now require await ...dispose().
- Documentation and all examples updated: resource management is always async and must be awaited; Disposable implementers may use both sync and async cleanup.
- Old-style, synchronous cleanup methods have been completely removed (API is now consistently async for all DI lifecycle management).
- Example and tutorial code now demonstrate async resource disposal patterns.

benchmark_di/pubspec.lock

@@ -47,7 +47,7 @@ packages:
       path: "../cherrypick"
       relative: true
     source: path
-    version: "3.0.0-dev.3"
+    version: "3.0.0-dev.5"
   collection:
     dependency: transitive
     description: