sprints and audit work

src/Policy/__Tests/StellaOps.Policy.Determinization.Tests/Models/ObservationDecayTests.cs

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+using FluentAssertions;
+using StellaOps.Policy.Determinization.Models;
+using Xunit;
+
+namespace StellaOps.Policy.Determinization.Tests.Models;
+
+public class ObservationDecayTests
+{
+    [Fact]
+    public void Fresh_Should_CreateZeroAgeDecay()
+    {
+        // Arrange
+        var now = DateTimeOffset.UtcNow;
+
+        // Act
+        var decay = ObservationDecay.Fresh(now);
+
+        // Assert
+        decay.ObservedAt.Should().Be(now);
+        decay.RefreshedAt.Should().Be(now);
+        decay.CalculateDecay(now).Should().Be(1.0);
+    }
+
+    [Fact]
+    public void CalculateDecay_Should_ApplyHalfLifeFormula()
+    {
+        // Arrange
+        var observedAt = new DateTimeOffset(2026, 1, 1, 0, 0, 0, TimeSpan.Zero);
+        var decay = ObservationDecay.Create(observedAt, observedAt);
+
+        // After 14 days (one half-life), decay should be ~0.5
+        var after14Days = observedAt.AddDays(14);
+
+        // Act
+        var decayValue = decay.CalculateDecay(after14Days);
+
+        // Assert
+        decayValue.Should().BeApproximately(0.5, 0.01);
+    }
+
+    [Fact]
+    public void CalculateDecay_Should_NotDropBelowFloor()
+    {
+        // Arrange
+        var observedAt = new DateTimeOffset(2026, 1, 1, 0, 0, 0, TimeSpan.Zero);
+        var decay = ObservationDecay.Create(observedAt, observedAt);
+
+        // Very old observation (1 year)
+        var afterYear = observedAt.AddDays(365);
+
+        // Act
+        var decayValue = decay.CalculateDecay(afterYear);
+
+        // Assert
+        decayValue.Should().BeGreaterThanOrEqualTo(decay.Floor);
+    }
+
+    [Fact]
+    public void IsStale_Should_DetectStaleObservations()
+    {
+        // Arrange
+        var observedAt = new DateTimeOffset(2026, 1, 1, 0, 0, 0, TimeSpan.Zero);
+        var decay = ObservationDecay.Create(observedAt, observedAt);
+
+        // Decay drops below 0.5 threshold around 14 days
+        var before = observedAt.AddDays(10);
+        var after = observedAt.AddDays(20);
+
+        // Act & Assert
+        decay.IsStale(before).Should().BeFalse();
+        decay.IsStale(after).Should().BeTrue();
+    }
+
+    [Fact]
+    public void CalculateDecay_Should_ReturnOneForFutureDates()
+    {
+        // Arrange
+        var now = DateTimeOffset.UtcNow;
+        var decay = ObservationDecay.Fresh(now);
+
+        // Act (future date, should not decay)
+        var futureDecay = decay.CalculateDecay(now.AddDays(-1));
+
+        // Assert
+        futureDecay.Should().Be(1.0);
+    }
+}