fix: Windows兼容性修复 + P1-24弱模型优化 + 3.2.9b禁止降级方案 + AirRvr三层审查放行标准

- lock.py: 跨平台进程锁（Unix fcntl / Windows msvcrt / O_CREAT|O_EXCL降级）
- eng_mode.py/eng_orchestrator.py: hasattr(os, "getloadavg") Windows防护
- arc_mode.py: 路径分隔符 replace("\\", "/") Windows兼容
- deploy_runtime.py: 修复语法错误（清理 import tempfile 残留）
- P1-24(3.2.18): AMBIGUOUS_VERBS歧义词检测 + SAFE_VERBS安全动词 + validate_task_description()
- TaskNode.keep_constraints 保留约束字段 + JSON序列化
- _build_graph_from_todo 返回歧义警告 + Arc自检集成
- 3.2.9b: FORBIDDEN_DEGRADATION_PATTERNS + check_forbidden_degradation()
- AirRvr三层审查放行标准: ReviewVerdict + evaluate_review_pass() + is_forbidden_pass_reason()
- commands/arc.md: 弱模型安全重写（操作类型拆分+保留约束+自检）
- commands/do.md/eng.md/rvr.md: 禁止降级方案 + 三层审查标准
- 测试: 7个新测试 + 74全量通过

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

lib/air_runtime/modes/eng_mode.py

@@ -370,13 +370,15 @@ def monitor_engine(project_root: Path) -> dict:
         else:
             ready_to_merge += 1 if worker.get("status") == "done" else 0
 
-    # 资源压力检测
-    try:
-        load = os.getloadavg()[0]
-        cpu_count = os.cpu_count() or 4
-        resource_pressure = load > cpu_count * 2
-    except OSError:
-        resource_pressure = False
+    # 资源压力检测 (Unix only; Windows 上不可用)
+    resource_pressure = False
+    if hasattr(os, "getloadavg"):
+        try:
+            load = os.getloadavg()[0]
+            cpu_count = os.cpu_count() or 4
+            resource_pressure = load > cpu_count * 2
+        except OSError:
+            pass
 
     # P1-21: ADR 变更自动检测
     adr_changes = _detect_adr_changes(project_root, state)