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>

test_p1_21_supplement.py

@@ -228,7 +228,7 @@ def test_eng_monitor_detects_adr_change():
         from air_runtime.modes.eng_mode import _paths
         paths = _paths(project_root)
         state = safe_json_load(paths["state"]) or {}
-        changes = _detect_adr_changes(project_root, state)
+        changes, _watcher = _detect_adr_changes(project_root, state)
         assert len(changes) == 0  # 首次只初始化，无变更
 
         # 保存 hash 到 state
@@ -240,7 +240,7 @@ def test_eng_monitor_detects_adr_change():
 
         # 第二次检测
         state = safe_json_load(paths["state"]) or {}
-        changes = _detect_adr_changes(project_root, state)
+        changes, _watcher = _detect_adr_changes(project_root, state)
         assert len(changes) == 1
         assert changes[0].adr_id == "ADR-0005"
         assert changes[0].kind == "superseded"