fix: 强化 CPU/memory checker 错误处理、timeout 遵守和快照校验

- Memory checker: reader 与 ctx.signal race，abort 返回 memory/timeout，reject 保持 memory/snapshot
- CPU checker: 第二次快照异常返回 cpu/snapshot，计算前校验空数组/核心数不一致/非有限值/负 delta
- CPU 计算: 零 delta 安全处理，observation 不含 NaN/Infinity
- 文档: CPU 互补描述修正，Memory timeout 约束说明
- 测试: +18 覆盖 timeout、异常和边界输入

tests/server/checker/runner/memory/execute.test.ts

@@ -153,6 +153,66 @@ describe("MemoryChecker execute", () => {
     expect(result.observation).toBeNull();
   });
 
+  test("signal 已 abort 时返回 timeout failure", async () => {
+    const reader = () => Promise.resolve(makeMemData());
+    const checker = new MemoryChecker(reader);
+
+    const target: RawTargetConfig = { id: "mem-test", memory: {}, type: "memory" };
+    const resolved = checker.resolve(target, makeResolveContext());
+
+    const controller = new AbortController();
+    controller.abort();
+    const result = await checker.execute(resolved, { signal: controller.signal });
+
+    expect(result.matched).toBe(false);
+    expect(result.failure?.phase).toBe("memory");
+    expect(result.failure?.path).toBe("timeout");
+    expect(result.observation).toBeNull();
+  });
+
+  test("pending reader 被 signal abort 后返回 timeout failure", async () => {
+    const reader = () =>
+      new Promise<Systeminformation.MemData>(() => {
+        // 故意永不 resolve，模拟悬挂的 reader
+      });
+    const checker = new MemoryChecker(reader);
+
+    const target: RawTargetConfig = { id: "mem-test", memory: {}, type: "memory" };
+    const resolved = checker.resolve(target, makeResolveContext());
+
+    const controller = new AbortController();
+    const executePromise = checker.execute(resolved, { signal: controller.signal });
+
+    controller.abort();
+
+    const result = await executePromise;
+
+    expect(result.matched).toBe(false);
+    expect(result.failure?.phase).toBe("memory");
+    expect(result.failure?.path).toBe("timeout");
+    expect(result.observation).toBeNull();
+  });
+
+  test("reader 在 abort 前 resolve 时返回正常结果", async () => {
+    const data = makeMemData({ active: 4294967296, total: 8589934592 });
+    const reader = () => Promise.resolve(data);
+    const checker = new MemoryChecker(reader);
+
+    const target: RawTargetConfig = { id: "mem-test", memory: {}, type: "memory" };
+    const resolved = checker.resolve(target, makeResolveContext());
+    resolved.expect = { usagePercent: { lte: 85 } };
+
+    const ctx = { signal: new AbortController().signal };
+    const result = await checker.execute(resolved, ctx);
+
+    expect(result.matched).toBe(true);
+    expect(result.failure).toBeNull();
+    expect(result.observation).toMatchObject({
+      totalBytes: 8589934592,
+      usagePercent: 50,
+    });
+  });
+
   test("detail 格式", async () => {
     const data = makeMemData({ active: 4294967296, total: 8589934592 });
     const reader = () => Promise.resolve(data);