feat: 添加 Session 亲和路由支持（NewSession / WithAffinity）

- 新增 NewSession(pool)：返回固定到同一 worker 进程的 Pool 视图
- 新增 WithAffinity(ctx, key)：相同 key 通过 FNV hash 稳定路由到同一 worker
- pool.acquire 支持亲和路由，无亲和时保持轮询
- example 添加 Session / WithAffinity / 全局变量共享三组演示

example/worker.py

@@ -143,6 +143,45 @@ def get_user_via_go(uid: int) -> dict:
     return dataclasses.asdict(user)
 
 
+# ── Session 亲和示例 ──────────────────────────────────────────────────────────
+# _sessions 保存每个 session 的状态，key 由调用方提供
+_sessions: dict = {}
+
+# _global_counter 是进程级全局变量，同一 worker 的所有 session 共享
+_global_counter: int = 0
+
+
+@expose
+def global_increment(delta: int) -> str:
+    global _global_counter
+    _global_counter += delta
+    return f"[worker {worker_id}] counter = {_global_counter}"
+
+
+@expose
+def global_get() -> str:
+    return f"[worker {worker_id}] counter = {_global_counter}"
+
+
+@expose
+def session_init(session_id: str, value: int) -> str:
+    _sessions[session_id] = {"value": value, "steps": []}
+    return f"[worker {worker_id}] session {session_id} init with {value}"
+
+
+@expose
+def session_step(session_id: str, delta: int) -> int:
+    s = _sessions[session_id]
+    s["value"] += delta
+    s["steps"].append(delta)
+    return s["value"]
+
+
+@expose
+def session_result(session_id: str) -> dict:
+    return _sessions.pop(session_id)
+
+
 if __name__ == "__main__":
     run()
     print("worker_id", worker_id)