性能优化: 热路径零分配, 自适应池CAS无锁化, 锁外计算

- port_scan: fmt.Sprintf→JoinHostPort+fmtPort零分配地址格式化 (2.5x)
- port_scan: strings.ToLower→containsFold零分配大小写不敏感匹配 (2.5x)
- port_scan: slidingWindowSchedule修复semaphore泄漏bug
- service_probe: readFromConn预分配4KB缓冲区消除扩容
- adaptive_pool: maybeAdjust用atomic CAS代替持锁检查, 99%免锁
- adaptive_timeout: Timeout锁外计算均值/标准差, 只锁缓存更新
- 新增perf_bench_test.go基准测试验证所有优化

core/port_scan_test.go

@@ -662,7 +662,7 @@ func TestIsResourceExhaustedError_EdgeCases(t *testing.T) {
 		{
 			name:     "大小写混合",
 			err:      fmt.Errorf("Too Many Open Files"),
-			expected: false, // 当前实现区分大小写
+			expected: true, // containsFold 不区分大小写
 		},
 		{
 			name:     "错误信息包含但不完全匹配",