feat(site-runtime): pure NextTagSubscribeInterval backoff step (double, ±20% jitter, clamp)

tests/ZB.MOM.WW.ScadaBridge.SiteRuntime.Tests/Actors/InstanceActorTagSubscribeBackoffTests.cs

@@ -0,0 +1,51 @@
+using ZB.MOM.WW.ScadaBridge.SiteRuntime.Actors;
+
+namespace ZB.MOM.WW.ScadaBridge.SiteRuntime.Tests.Actors;
+
+/// <summary>
+/// Lever 1 (2026-08-28): the pure backoff step behind the Instance Actor's tag-subscribe
+/// retry. Doubles with jitter, never below the floor, never above the ceiling; a ceiling
+/// below the floor degrades to a fixed interval rather than shrinking.
+/// </summary>
+public class InstanceActorTagSubscribeBackoffTests
+{
+    private static readonly TimeSpan Floor = TimeSpan.FromSeconds(5);
+    private static readonly TimeSpan Max = TimeSpan.FromSeconds(60);
+
+    [Fact]
+    public void Doubles_WithNoJitter()
+    {
+        var next = InstanceActor.NextTagSubscribeInterval(Floor, Floor, Max, jitterUnit: 0.5);
+        Assert.Equal(TimeSpan.FromSeconds(10), next);
+    }
+
+    [Fact]
+    public void Jitter_SpansPlusMinusTwentyPercent()
+    {
+        var low = InstanceActor.NextTagSubscribeInterval(Floor, Floor, Max, jitterUnit: 0.0);
+        var high = InstanceActor.NextTagSubscribeInterval(Floor, Floor, Max, jitterUnit: 1.0);
+        Assert.Equal(TimeSpan.FromSeconds(8), low);   // 10 s × 0.8
+        Assert.Equal(TimeSpan.FromSeconds(12), high); // 10 s × 1.2
+    }
+
+    [Fact]
+    public void NeverExceedsCeiling_EvenWithPositiveJitter()
+    {
+        var next = InstanceActor.NextTagSubscribeInterval(TimeSpan.FromSeconds(40), Floor, Max, jitterUnit: 1.0);
+        Assert.Equal(Max, next);
+    }
+
+    [Fact]
+    public void NeverBelowFloor_EvenWithNegativeJitter()
+    {
+        var next = InstanceActor.NextTagSubscribeInterval(TimeSpan.FromSeconds(1), Floor, Max, jitterUnit: 0.0);
+        Assert.True(next >= Floor, $"{next} < floor");
+    }
+
+    [Fact]
+    public void CeilingBelowFloor_DegradesToFixedFloor()
+    {
+        var next = InstanceActor.NextTagSubscribeInterval(Floor, Floor, TimeSpan.FromSeconds(1), jitterUnit: 0.5);
+        Assert.Equal(Floor, next);
+    }
+}