refactor: 重构项目代码结构，拆分职责模块

1. 抽离图组合逻辑到pyflowx.compose，原graph.py仅保留单图DAG逻辑
2. 抽离命令执行逻辑到pyflowx.command，移除task.py内的_run_command
3. 重构上下文签名缓存，优化性能
4. 移除废弃的utils.perf_timer相关代码
5. 为JSONBackend添加batch批量落盘优化
6. 调整导入路径与公开API，更新测试用例
7. 简化条件判断逻辑，移除冗余代码

tests/test_graph.py

@@ -5,8 +5,8 @@ from __future__ import annotations
 import pytest
 
 import pyflowx as px
+from pyflowx.compose import GraphComposer, compose
 from pyflowx.errors import CycleError, DuplicateTaskError, MissingDependencyError
-from pyflowx.graph import GraphComposer, compose
 
 
 def _fn() -> None:

tests/test_task.py

@@ -345,14 +345,14 @@ def test_task_result_default_status() -> None:
 
 
 # ---------------------------------------------------------------------- #
-# _run_command callable 命令测试
+# run_command callable 命令测试
 # ---------------------------------------------------------------------- #
 def test_run_command_callable_verbose_with_cwd(capsys: pytest.CaptureFixture[str], tmp_path: Path) -> None:
     """callable 命令 verbose 模式应打印信息."""
-    spec = TaskSpec("a", cmd=lambda: "result", verbose=True, cwd=tmp_path)
-    import pyflowx.task as task_module
+    from pyflowx.command import run_command
 
-    result = task_module._run_command(spec)
+    spec = TaskSpec("a", cmd=lambda: "result", verbose=True, cwd=tmp_path)
+    result = run_command(spec)
     assert result == "result"
     captured = capsys.readouterr()
     assert "执行可调用命令" in captured.out
@@ -361,8 +361,8 @@ def test_run_command_callable_verbose_with_cwd(capsys: pytest.CaptureFixture[str
 
 def test_run_command_callable_exception() -> None:
     """callable 命令抛异常应转为 RuntimeError."""
-    spec = TaskSpec("a", cmd=lambda: (_ for _ in ()).throw(RuntimeError("callable error")))
-    import pyflowx.task as task_module
+    from pyflowx.command import run_command
 
+    spec = TaskSpec("a", cmd=lambda: (_ for _ in ()).throw(RuntimeError("callable error")))
     with pytest.raises(RuntimeError, match="可调用命令执行异常"):
-        task_module._run_command(spec)
+        run_command(spec)

tests/test_utils.py

@@ -1,65 +0,0 @@
-import time
-
-import pytest
-from pytest_mock import MockerFixture
-
-from pyflowx.utils import _perf_metrics, perf_timer
-
-
-@pytest.fixture(autouse=True)
-def reset_perf_metrics():
-    """重置性能指标."""
-    _perf_metrics.clear()
-
-
-class TestPerformanceTimer:
-    def test_perf_timer(self):
-
-        @perf_timer()
-        def test_func():
-            time.sleep(0.1)
-
-        test_func()
-
-        assert _perf_metrics["test_func"] is not None
-        assert _perf_metrics["test_func"]["count"] == 1
-        assert _perf_metrics["test_func"]["total_time"] >= 0.1
-
-    def test_perf_timer_report(self, mocker: MockerFixture):
-        mock_log = mocker.patch("logging.info")
-
-        @perf_timer(report=True, unit="ms", precision=3)
-        def test_func():
-            time.sleep(0.1)
-
-        test_func()
-
-        assert _perf_metrics["test_func"] is not None
-        assert _perf_metrics["test_func"]["count"] == 1
-        assert _perf_metrics["test_func"]["total_time"] >= 0.1
-
-        assert mock_log.call_count == 1
-
-    def test_generate_report(self, mocker: MockerFixture, caplog: pytest.LogCaptureFixture):
-        mock_log = mocker.patch("logging.info")
-
-        from pyflowx.utils import _generate_report
-
-        @perf_timer(report=True, unit="ms", precision=3)
-        def test_func():
-            time.sleep(0.1)
-
-        @perf_timer(report=True, unit="ms", precision=3)
-        def test_func2():
-            time.sleep(0.2)
-
-        test_func()
-        test_func2()
-
-        _generate_report("ms", 3)
-
-        assert mock_log.call_count == 3
-        assert _perf_metrics["test_func"]["count"] == 1
-        assert _perf_metrics["test_func"]["total_time"] >= 0.1
-        assert _perf_metrics["test_func2"]["count"] == 1
-        assert _perf_metrics["test_func2"]["total_time"] >= 0.2