feat: 3.2.17中途变更处理(git驱动) + 3.2.9a Worker git强制 + 3.2.5 Phase7 squash merge + Arc git初始化检测

- 新增 ChangeClassifier（爆炸半径分类：IMPLEMENTATION/INTERFACE/GLOBAL_CONSTRAINT）
- 新增 ImpactPropagator（BFS影响传播：IMPACTED/BOUNDARY/SAFE差异化标记）
- task_graph.py：invalidate_by_adr()差异化失效 + CascadeReport扩展字段（向后兼容）
- eng_mode.py：三阶段差异化流程（分类→传播→失效→git操作→验证任务→重规划）
- eng_mode.py：_git_squash_merge_and_tag() + Phase 7 集成
- do_mode.py：_ensure_all_committed() Worker git操作强制
- adr_watcher.py：内容快照 + get_content_for_classification()
- events.py：ADR_CLASSIFIED/IMPACT_PROPAGATED/BOUNDARY_VERIFICATION_GENERATED
- partial_replanner.py：replan_with_constraints() + generate_verification_tasks()
- project_bootstrap.py：ensure_git_initialized() Arc规划前检测
- test_p1_21_phase2.py：21个新测试，95个全量测试0失败

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

lib/air_runtime/impact_propagator.py

@@ -0,0 +1,104 @@
+"""
+BFS 影响传播 — P1-21 / 3.2.17 Phase 1 组件。
+根据 BlastRadius 在任务 DAG 中传播影响，标记每个任务为 IMPACTED / BOUNDARY / SAFE。
+"""
+
+from __future__ import annotations
+
+import enum
+from collections import deque
+from dataclasses import dataclass, field
+
+from air_runtime.change_classifier import BlastRadius
+from air_runtime.task_graph import TaskGraph
+
+
+class ImpactLabel(enum.Enum):
+    IMPACTED = "impacted"
+    BOUNDARY = "boundary"
+    SAFE = "safe"
+
+
+@dataclass
+class PropagationResult:
+    impacted: list[str] = field(default_factory=list)
+    boundary: list[str] = field(default_factory=list)
+    safe: list[str] = field(default_factory=list)
+
+
+class ImpactPropagator:
+    """BFS 影响传播，基于 BlastRadius 差异化标记。
+
+    传播规则：
+    - IMPLEMENTATION: 仅直接引用 ADR 的任务 → IMPACTED，其他 → SAFE（不下游传播）
+    - INTERFACE: 直接引用 → IMPACTED，下游 1 跳邻居 → BOUNDARY，其他 → SAFE
+    - GLOBAL_CONSTRAINT: 全图 → IMPACTED
+
+    结果存储在 TaskNode.meta["impact_label"]，不修改 status 字段（正交）。
+    """
+
+    _META_KEY = "impact_label"
+
+    def propagate(self, graph: TaskGraph, adr_id: str,
+                  blast_radius: BlastRadius) -> PropagationResult:
+        self.clear_labels(graph)
+
+        tasks_ref_adr = [n.id for n in graph.nodes.values() if adr_id in n.adr_refs]
+        all_ids = set(graph.nodes.keys())
+
+        if blast_radius == BlastRadius.GLOBAL_CONSTRAINT:
+            for nid in all_ids:
+                graph.nodes[nid].meta[self._META_KEY] = ImpactLabel.IMPACTED.value
+            return PropagationResult(
+                impacted=list(all_ids), boundary=[], safe=[],
+            )
+
+        if blast_radius == BlastRadius.IMPLEMENTATION:
+            impacted = list(tasks_ref_adr)
+            safe = [nid for nid in all_ids if nid not in impacted]
+            for nid in impacted:
+                graph.nodes[nid].meta[self._META_KEY] = ImpactLabel.IMPACTED.value
+            for nid in safe:
+                graph.nodes[nid].meta[self._META_KEY] = ImpactLabel.SAFE.value
+            return PropagationResult(impacted=impacted, boundary=[], safe=safe)
+
+        # INTERFACE: direct refs → IMPACTED, 1-hop downstream → BOUNDARY, rest → SAFE
+        impacted = set(tasks_ref_adr)
+
+        # BFS 收集下游 1 跳邻居（仅从 IMPACTED 出发，排除自身）
+        boundary = set()
+        for nid in list(impacted):
+            node = graph.nodes.get(nid)
+            if node:
+                for target in node.out_edges:
+                    if target not in impacted:
+                        boundary.add(target)
+
+        safe = all_ids - impacted - boundary
+
+        for nid in impacted:
+            graph.nodes[nid].meta[self._META_KEY] = ImpactLabel.IMPACTED.value
+        for nid in boundary:
+            graph.nodes[nid].meta[self._META_KEY] = ImpactLabel.BOUNDARY.value
+        for nid in safe:
+            graph.nodes[nid].meta[self._META_KEY] = ImpactLabel.SAFE.value
+
+        return PropagationResult(
+            impacted=list(impacted), boundary=list(boundary), safe=list(safe),
+        )
+
+    def impacted_tasks(self, graph: TaskGraph) -> list[str]:
+        return [nid for nid, n in graph.nodes.items()
+                if n.meta.get(self._META_KEY) == ImpactLabel.IMPACTED.value]
+
+    def boundary_tasks(self, graph: TaskGraph) -> list[str]:
+        return [nid for nid, n in graph.nodes.items()
+                if n.meta.get(self._META_KEY) == ImpactLabel.BOUNDARY.value]
+
+    def safe_tasks(self, graph: TaskGraph) -> list[str]:
+        return [nid for nid, n in graph.nodes.items()
+                if n.meta.get(self._META_KEY) == ImpactLabel.SAFE.value]
+
+    def clear_labels(self, graph: TaskGraph) -> None:
+        for node in graph.nodes.values():
+            node.meta.pop(self._META_KEY, None)