feat: P1-21 补充 — ADRWatcher 自动检测 + PartialReplanner 局部重规划

- 新增 adr_watcher.py: SHA256 hash 监控 ADR 文件变更
  - snapshot() 初始快照, detect_changes() 增量检测
  - 识别 new/modified/superseded/deleted 四种变更类型
  - _parse_status() 解析 ADR Status 字段
- 新增 partial_replanner.py: 局部重规划器
  - replan() 仅生成受影响任务的替代计划
  - _extract_stable_interfaces() 提取未受影响 DONE 任务接口约束
  - 产出 replan-request.json 供 Arc 读取
- Eng monitor_engine 集成 ADR 检测: 每轮轮询调用 _detect_adr_changes()
- Eng handle_adr_invalidation 集成 PartialReplanner 输出
- events.py 新增 ADR 变更相关事件常量
- task_graph.py apply_full_replace 保留 DISPATCHED, 不保留 INVALIDATED
- 10 项补充测试全通过，含完整 ADR 流程场景

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/air_runtime/adr_watcher.py

@@ -0,0 +1,109 @@
+"""
+ADR 文件变更监控 — P1-21 ADR 变更自动检测。
+基于 SHA256 hash 对比检测 ADR 文件变更，自动触发级联失效。
+"""
+
+from __future__ import annotations
+
+import hashlib
+import re
+from dataclasses import dataclass, field
+from pathlib import Path
+
+
+@dataclass
+class ADRChange:
+    """ADR 文件变更记录。"""
+    adr_id: str
+    kind: str  # "new" | "superseded" | "modified"
+    path: str = ""
+    old_hash: str = ""
+    new_hash: str = ""
+
+
+class ADRWatcher:
+    """监控 ADR 文件变更，自动触发级联失效。
+
+    AirEng 在每轮轮询时调用 detect_changes()，
+    发现 superseded 或 modified 变更时自动触发 invalidate_by_adr()。
+    """
+
+    def __init__(self, adr_dir: Path):
+        self._adr_dir = adr_dir
+        self._known_hashes: dict[str, str] = {}
+
+    def snapshot(self) -> None:
+        """启动时记录所有 ADR 的内容 hash。"""
+        if not self._adr_dir.exists():
+            return
+        for adr_file in sorted(self._adr_dir.glob("ADR-*.md")):
+            adr_id = self._extract_adr_id(adr_file)
+            self._known_hashes[adr_id] = hashlib.sha256(
+                adr_file.read_bytes()
+            ).hexdigest()
+
+    def detect_changes(self) -> list[ADRChange]:
+        """对比当前 ADR hash 与已知 hash，返回变更列表。"""
+        if not self._adr_dir.exists():
+            return []
+
+        changes: list[ADRChange] = []
+        seen_ids: set[str] = set()
+
+        for adr_file in sorted(self._adr_dir.glob("ADR-*.md")):
+            adr_id = self._extract_adr_id(adr_file)
+            seen_ids.add(adr_id)
+            current_hash = hashlib.sha256(adr_file.read_bytes()).hexdigest()
+            old_hash = self._known_hashes.get(adr_id)
+
+            if old_hash is None:
+                changes.append(ADRChange(
+                    adr_id=adr_id, kind="new",
+                    path=str(adr_file), old_hash="", new_hash=current_hash,
+                ))
+            elif current_hash != old_hash:
+                status = self._parse_status(adr_file)
+                if status == "superseded":
+                    changes.append(ADRChange(
+                        adr_id=adr_id, kind="superseded",
+                        path=str(adr_file), old_hash=old_hash, new_hash=current_hash,
+                    ))
+                else:
+                    changes.append(ADRChange(
+                        adr_id=adr_id, kind="modified",
+                        path=str(adr_file), old_hash=old_hash, new_hash=current_hash,
+                    ))
+            self._known_hashes[adr_id] = current_hash
+
+        # 检查被删除的 ADR
+        for adr_id in list(self._known_hashes.keys()):
+            if adr_id not in seen_ids:
+                changes.append(ADRChange(
+                    adr_id=adr_id, kind="deleted",
+                    path="", old_hash=self._known_hashes[adr_id], new_hash="",
+                ))
+                del self._known_hashes[adr_id]
+
+        return changes
+
+    @staticmethod
+    def _extract_adr_id(adr_file: Path) -> str:
+        """从文件名提取 ADR ID，如 'ADR-0005-ffmpeg-decode.md' → 'ADR-0005'。"""
+        match = re.match(r"(ADR-\d+)", adr_file.stem)
+        if match:
+            return match.group(1)
+        return adr_file.stem
+
+    @staticmethod
+    def _parse_status(adr_file: Path) -> str:
+        """解析 ADR 文件中的 Status 字段。"""
+        try:
+            text = adr_file.read_text(encoding="utf-8")
+        except (OSError, UnicodeDecodeError):
+            return "unknown"
+        for line in text.splitlines():
+            lower = line.lower().strip()
+            if lower.startswith("status:") or lower.startswith("status :"):
+                status = line.split(":", 1)[1].strip().lower()
+                return status
+        return "unknown"