chore: initial project import

backend/shared/Monitor.Measurement/Core/MonoClock.cs

@@ -0,0 +1,26 @@
+namespace Monitor.Measurement;
+
+/// <summary>节点的单调时钟（微秒）：t1…t4、t_send、t_recv 都用它；Probe 和 Agent 各自一个，块头里的时钟锚点把它对应到 UTC。</summary>
+public sealed class MonoClock
+{
+    private readonly TimeProvider _time;
+
+    public MonoClock(TimeProvider time)
+    {
+        _time = time;
+    }
+
+    public TimeProvider Time => _time;
+
+    public long NowUs()
+    {
+        long ticks = _time.GetTimestamp();
+        // 先除后乘会丢精度，先乘再除在 64 位内不会溢出（频率通常是 1e9 或 1e7）
+        return (ticks / _time.TimestampFrequency * 1_000_000) + (ticks % _time.TimestampFrequency * 1_000_000 / _time.TimestampFrequency);
+    }
+
+    public ClockAnchor Anchor() => new(NowUs(), _time.GetUtcNow().ToUnixTimeMilliseconds());
+
+    /// <summary>单调时钟读数 <paramref name="thenUs"/> 对应的 UTC 毫秒（只用于块的粗略起点，精确时间由块头的时钟锚点换算）。</summary>
+    public long UtcMsAt(long thenUs) => _time.GetUtcNow().ToUnixTimeMilliseconds() - (Math.Max(0, NowUs() - thenUs) / 1000);
+}