perf(render): bound animation and alpha scratch residency

Complete Slice D3 by replacing the unbounded animation dictionary with a concurrent byte/count LRU and by putting the three retail alpha scratch owners behind one typed aggregate budget. Preserve immediate growth and draw order while reclaiming one-frame density spikes after sustained under-use. Close stale bounds-cache issue evidence without inventing a cache.

tests/AcDream.App.Tests/Rendering/Residency/RetainedScratchCapacityPolicyTests.cs

@@ -0,0 +1,86 @@
+using AcDream.App.Rendering.Residency;
+
+namespace AcDream.App.Tests.Rendering.Residency;
+
+public sealed class RetainedScratchCapacityPolicyTests
+{
+    [Fact]
+    public void AlphaBudgetProfilePartitionsTheAggregateExactly()
+    {
+        AlphaScratchBudgetProfile profile =
+            AlphaScratchBudgetProfile.Create(16 * 1024 * 1024);
+
+        Assert.Equal(4 * 1024 * 1024, profile.QueueBytes);
+        Assert.Equal(8 * 1024 * 1024, profile.DispatcherBytes);
+        Assert.Equal(4 * 1024 * 1024, profile.ParticleBytes);
+        Assert.Equal(16 * 1024 * 1024, profile.TotalBytes);
+    }
+
+    [Fact]
+    public void SpikeRetainsWarmCapacityUntilDemandFallsThenConverges()
+    {
+        var policy = new RetainedScratchCapacityPolicy(
+            budgetBytes: 1024,
+            lowDemandSamplesBeforeShrink: 3);
+
+        Assert.Equal(
+            4096,
+            policy.ObserveAndSelectCapacity(
+                currentCapacity: 4096,
+                requiredCapacity: 4096,
+                bytesPerUnit: 4,
+                minimumCapacity: 64,
+                growthQuantum: 64));
+        Assert.Equal(4096, ObserveLowDemand(policy));
+        Assert.Equal(4096, ObserveLowDemand(policy));
+
+        int converged = ObserveLowDemand(policy);
+
+        Assert.InRange(converged, 64, 256);
+        Assert.True((long)converged * 4 <= 1024);
+    }
+
+    [Fact]
+    public void SustainedLegitimateDemandIsNeverTrimmedBelowRequirement()
+    {
+        var policy = new RetainedScratchCapacityPolicy(
+            budgetBytes: 1024,
+            lowDemandSamplesBeforeShrink: 2);
+
+        for (int i = 0; i < 10; i++)
+        {
+            Assert.Equal(
+                4096,
+                policy.ObserveAndSelectCapacity(
+                    currentCapacity: 4096,
+                    requiredCapacity: 4096,
+                    bytesPerUnit: 4,
+                    minimumCapacity: 64,
+                    growthQuantum: 64));
+        }
+    }
+
+    [Fact]
+    public void RequiredGrowthIsImmediateAndQuantumRounded()
+    {
+        var policy = new RetainedScratchCapacityPolicy(1024);
+
+        Assert.Equal(
+            320,
+            policy.ObserveAndSelectCapacity(
+                currentCapacity: 64,
+                requiredCapacity: 257,
+                bytesPerUnit: 4,
+                minimumCapacity: 64,
+                growthQuantum: 64));
+    }
+
+    private static int ObserveLowDemand(
+        RetainedScratchCapacityPolicy policy) =>
+        policy.ObserveAndSelectCapacity(
+            currentCapacity: 4096,
+            requiredCapacity: 32,
+            bytesPerUnit: 4,
+            minimumCapacity: 64,
+            growthQuantum: 64);
+}