feat(headless): add deterministic multi-session scheduler

src/AcDream.Headless/Hosting/HeadlessMonotonicTime.cs

@@ -0,0 +1,25 @@
+namespace AcDream.Headless.Hosting;
+
+internal static class HeadlessMonotonicTime
+{
+    internal static long Add(
+        TimeProvider timeProvider,
+        long timestamp,
+        TimeSpan duration)
+    {
+        ArgumentNullException.ThrowIfNull(timeProvider);
+        if (duration < TimeSpan.Zero)
+            throw new ArgumentOutOfRangeException(nameof(duration));
+
+        double ticks =
+            duration.TotalSeconds * timeProvider.TimestampFrequency;
+        if (!double.IsFinite(ticks) || ticks > long.MaxValue)
+            return long.MaxValue;
+        long delta = Math.Max(
+            1L,
+            checked((long)Math.Ceiling(ticks)));
+        return timestamp > long.MaxValue - delta
+            ? long.MaxValue
+            : timestamp + delta;
+    }
+}