feat: 初始化PyFlowX轻量级DAG任务调度库

实现完整的DAG任务调度核心功能，包括：
1.  支持同步/异步/线程三种执行策略
2.  自动上下文注入，无需手动绑定任务依赖
3.  内置状态后端，支持断点续跑
4.  提供完整的测试用例与示例代码
5.  添加CI/CD配置与发布流程

tests/test_graph.py

@@ -0,0 +1,131 @@
+"""Tests for Graph construction, validation, layering and subgraphs."""
+
+from __future__ import annotations
+
+import pytest
+
+import pyflowx as px
+from pyflowx.errors import CycleError, DuplicateTaskError, MissingDependencyError
+
+
+def _fn() -> None:
+    return None
+
+
+def test_from_specs_builds_graph() -> None:
+    graph = px.Graph.from_specs([
+        px.TaskSpec("a", _fn),
+        px.TaskSpec("b", _fn, ("a",)),
+        px.TaskSpec("c", _fn, ("a", "b")),
+    ])
+    assert set(graph.names) == {"a", "b", "c"}
+    assert graph.dependencies("c") == ("a", "b")
+    assert len(graph) == 3
+    assert "a" in graph
+
+
+def test_from_specs_allows_forward_references() -> None:
+    # b depends on a, but a is declared after b — order should not matter.
+    graph = px.Graph.from_specs([
+        px.TaskSpec("b", _fn, ("a",)),
+        px.TaskSpec("a", _fn),
+    ])
+    assert graph.layers() == [["a"], ["b"]]
+
+
+def test_duplicate_task_raises() -> None:
+    with pytest.raises(DuplicateTaskError):
+        px.Graph.from_specs([
+            px.TaskSpec("a", _fn),
+            px.TaskSpec("a", _fn),
+        ])
+
+
+def test_missing_dependency_raises() -> None:
+    with pytest.raises(MissingDependencyError) as exc_info:
+        px.Graph.from_specs([px.TaskSpec("b", _fn, ("a",))])
+    assert exc_info.value.task == "b"
+    assert exc_info.value.dependency == "a"
+
+
+def test_cycle_detection() -> None:
+    with pytest.raises(CycleError):
+        px.Graph.from_specs([
+            px.TaskSpec("a", _fn, ("c",)),
+            px.TaskSpec("b", _fn, ("a",)),
+            px.TaskSpec("c", _fn, ("b",)),
+        ])
+
+
+def test_layers_grouping() -> None:
+    graph = px.Graph.from_specs([
+        px.TaskSpec("a", _fn),
+        px.TaskSpec("b", _fn),
+        px.TaskSpec("c", _fn, ("a", "b")),
+        px.TaskSpec("d", _fn, ("c",)),
+    ])
+    layers = graph.layers()
+    assert layers == [["a", "b"], ["c"], ["d"]]
+
+
+def test_self_dependency_rejected() -> None:
+    with pytest.raises(ValueError):
+        px.TaskSpec("a", _fn, ("a",))
+
+
+def test_to_mermaid() -> None:
+    graph = px.Graph.from_specs([
+        px.TaskSpec("a", _fn),
+        px.TaskSpec("b", _fn, ("a",)),
+    ])
+    mermaid = graph.to_mermaid()
+    assert mermaid.startswith("graph TD")
+    assert 'a["a"]' in mermaid
+    assert "a --> b" in mermaid
+
+
+def test_to_mermaid_invalid_orientation() -> None:
+    graph = px.Graph.from_specs([px.TaskSpec("a", _fn)])
+    with pytest.raises(ValueError):
+        graph.to_mermaid("XX")
+
+
+def test_subgraph_by_tags() -> None:
+    graph = px.Graph.from_specs([
+        px.TaskSpec("a", _fn, tags=("ingest",)),
+        px.TaskSpec("b", _fn, ("a",), tags=("ingest",)),
+        px.TaskSpec("c", _fn, ("b",), tags=("report",)),
+    ])
+    sub = graph.subgraph(["ingest"])
+    assert set(sub.names) == {"a", "b"}
+    # Edge to dropped task c is removed; b no longer waits for anything
+    # outside the subgraph (c was never a dep of b anyway).
+    assert sub.dependencies("b") == ("a",)
+
+
+def test_subgraph_by_names() -> None:
+    graph = px.Graph.from_specs([
+        px.TaskSpec("a", _fn),
+        px.TaskSpec("b", _fn, ("a",)),
+        px.TaskSpec("c", _fn, ("b",)),
+    ])
+    sub = graph.subgraph_by_names(["a", "b"])
+    assert set(sub.names) == {"a", "b"}
+    # c is dropped, so b's dep on c (none here) — but a->b edge preserved.
+    assert sub.dependencies("b") == ("a",)
+
+
+def test_subgraph_by_names_unknown() -> None:
+    graph = px.Graph.from_specs([px.TaskSpec("a", _fn)])
+    with pytest.raises(KeyError):
+        graph.subgraph_by_names(["nope"])
+
+
+def test_describe() -> None:
+    graph = px.Graph.from_specs([
+        px.TaskSpec("a", _fn),
+        px.TaskSpec("b", _fn, ("a",)),
+    ])
+    desc = graph.describe()
+    assert "Layer 1" in desc
+    assert "Layer 2" in desc