feat: asyncio-native Actor framework with supervision, middleware, and pluggable mailbox

Lightweight actor library built on asyncio primitives (~800 lines):

- Actor base class with lifecycle hooks (on_started/on_stopped/on_restart)
- ActorRef with tell (fire-and-forget) and ask (request-response)
- Supervision: OneForOne/AllForOne strategies with restart limits
- Middleware pipeline for cross-cutting concerns
- Pluggable Mailbox interface (MemoryMailbox default, RedisMailbox optional)
- ReplyRegistry + ReplyChannel: ask() works across any mailbox backend
- System-level thread pool for blocking I/O (run_in_executor)
- Dead letter handling, poison message quarantine, parallel shutdown
- 22 tests + benchmark suite

backend/packages/harness/deerflow/actor/__init__.py

@@ -0,0 +1,40 @@
+"""Async Actor framework — lightweight, asyncio-native, supervision-ready.
+
+Usage::
+
+    from deerflow.actor import Actor, ActorSystem
+
+    class Greeter(Actor):
+        async def on_receive(self, message):
+            return f"Hello, {message}!"
+
+    async def main():
+        system = ActorSystem("app")
+        ref = await system.spawn(Greeter, "greeter")
+        reply = await ref.ask("World", timeout=5.0)
+        print(reply)  # Hello, World!
+        await system.shutdown()
+"""
+
+from .actor import Actor, ActorContext
+from .mailbox import Mailbox, MemoryMailbox
+from .middleware import Middleware
+from .ref import ActorRef, ReplyChannel
+from .supervision import AllForOneStrategy, Directive, OneForOneStrategy, SupervisorStrategy
+from .system import ActorSystem, DeadLetter
+
+__all__ = [
+    "Actor",
+    "ActorContext",
+    "ActorRef",
+    "ActorSystem",
+    "AllForOneStrategy",
+    "DeadLetter",
+    "Directive",
+    "Mailbox",
+    "MemoryMailbox",
+    "Middleware",
+    "OneForOneStrategy",
+    "ReplyChannel",
+    "SupervisorStrategy",
+]