feat: 新增多项核心功能并优化默认执行策略

1.  将CliRunner默认执行策略从sequential改为dependency
2.  新增RunReport的任务状态查询和时长统计方法
3.  实现task装饰器并补充executor参数文档
4.  新增进程池执行器支持CPU密集型任务
5.  新增Graph.chain链式构建和add_subgraph子图合并功能
6.  新增流式任务传递、进程池执行、命名空间等多类测试用例
7.  补充tests目录路径导入配置

tests/test_streaming.py

@@ -0,0 +1,63 @@
+"""Tests for streaming result passing (iterators between tasks)."""
+
+from __future__ import annotations
+
+from typing import Iterator
+
+import pyflowx as px
+
+
+def test_generator_passed_as_iterator() -> None:
+    """上游返回生成器，下游应能惰性消费."""
+
+    @px.task
+    def source() -> Iterator[int]:
+        yield from range(5)
+
+    @px.task(depends_on=("source",))
+    def consume(source: Iterator[int]) -> int:
+        return sum(source)
+
+    graph = px.Graph.from_specs([source, consume])
+    report = px.run(graph)
+    assert report.success
+    assert report["consume"] == 10
+
+
+def test_large_range_streaming() -> None:
+    """大范围迭代器流式传递，避免中间列表."""
+
+    @px.task
+    def numbers() -> Iterator[int]:
+        yield from range(1000)
+
+    @px.task(depends_on=("numbers",))
+    def total(numbers: Iterator[int]) -> int:
+        return sum(numbers)
+
+    graph = px.Graph.from_specs([numbers, total])
+    report = px.run(graph)
+    assert report.success
+    assert report["total"] == sum(range(1000))
+
+
+def test_chain_multiple_streams() -> None:
+    """多个流式任务串联."""
+
+    @px.task
+    def gen() -> Iterator[int]:
+        yield from range(10)
+
+    @px.task(depends_on=("gen",))
+    def doubled(gen: Iterator[int]) -> Iterator[int]:
+        for x in gen:
+            yield x * 2
+
+    @px.task(depends_on=("doubled",))
+    def collect(doubled: Iterator[int]) -> list[int]:
+        return list(doubled)
+
+    graph = px.Graph.from_specs([gen, doubled, collect])
+    report = px.run(graph)
+    assert report.success
+    assert report["collect"] == [x * 2 for x in range(10)]