feat(pipeline): MP0 - FrameStatsBuffer ring/percentile core

Fixed-capacity ring buffer of long samples with nearest-rank percentile
and max over the current window. Pure, allocation-free after
construction. Foundation for the MP0 frame profiler
(docs/superpowers/specs/2026-07-05-modern-pipeline-design.md).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

tests/AcDream.App.Tests/FrameStatsBufferTests.cs

@@ -0,0 +1,51 @@
+using AcDream.App.Diagnostics;
+using Xunit;
+
+namespace AcDream.App.Tests;
+
+public class FrameStatsBufferTests
+{
+    [Fact]
+    public void Percentiles_OnKnownDistribution_AreExact()
+    {
+        var buf = new FrameStatsBuffer(capacity: 100);
+        // 1..100 µs — p50 = 50, p95 = 95, p99 = 99, max = 100.
+        for (long i = 1; i <= 100; i++) buf.Push(i);
+
+        Assert.Equal(50, buf.Percentile(0.50));
+        Assert.Equal(95, buf.Percentile(0.95));
+        Assert.Equal(99, buf.Percentile(0.99));
+        Assert.Equal(100, buf.Max());
+    }
+
+    [Fact]
+    public void Push_PastCapacity_KeepsOnlyNewestWindow()
+    {
+        var buf = new FrameStatsBuffer(capacity: 4);
+        foreach (long v in new long[] { 1000, 1000, 1000, 1000, 1, 2, 3, 4 })
+            buf.Push(v);
+        // The four 1000s were overwritten; window is {1,2,3,4}.
+        Assert.Equal(4, buf.Count);
+        Assert.Equal(4, buf.Max());
+        Assert.Equal(2, buf.Percentile(0.50));
+    }
+
+    [Fact]
+    public void Percentile_Empty_ReturnsZero()
+    {
+        var buf = new FrameStatsBuffer(capacity: 8);
+        Assert.Equal(0, buf.Percentile(0.95));
+        Assert.Equal(0, buf.Max());
+        Assert.Equal(0, buf.Count);
+    }
+
+    [Fact]
+    public void Reset_ClearsWindow()
+    {
+        var buf = new FrameStatsBuffer(capacity: 8);
+        buf.Push(5); buf.Push(7);
+        buf.Reset();
+        Assert.Equal(0, buf.Count);
+        Assert.Equal(0, buf.Percentile(0.5));
+    }
+}