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>

src/AcDream.App/Diagnostics/FrameStatsBuffer.cs

@@ -0,0 +1,64 @@
+using System;
+
+namespace AcDream.App.Diagnostics;
+
+/// <summary>
+/// MP0 (2026-07-05) — fixed-capacity ring buffer of long samples
+/// (microseconds or bytes) with percentile/max over the current window.
+/// Pure and allocation-free after construction: <see cref="Percentile"/>
+/// sorts into a preallocated scratch array, so the 5-second report path
+/// allocates nothing. Not thread-safe — owned by the window loop thread.
+/// Spec: docs/superpowers/specs/2026-07-05-modern-pipeline-design.md §5.
+/// </summary>
+public sealed class FrameStatsBuffer
+{
+    private readonly long[] _samples;
+    private readonly long[] _scratch;
+    private int _cursor;
+    private int _count;
+
+    public FrameStatsBuffer(int capacity)
+    {
+        if (capacity <= 0) throw new ArgumentOutOfRangeException(nameof(capacity));
+        _samples = new long[capacity];
+        _scratch = new long[capacity];
+    }
+
+    public int Count => _count;
+
+    public void Push(long value)
+    {
+        _samples[_cursor] = value;
+        _cursor = (_cursor + 1) % _samples.Length;
+        if (_count < _samples.Length) _count++;
+    }
+
+    public void Reset()
+    {
+        _cursor = 0;
+        _count = 0;
+    }
+
+    /// <summary>
+    /// Nearest-rank percentile over the current window: element at
+    /// ceil(q·n) in the ascending sort (1-based), 0 when empty.
+    /// </summary>
+    public long Percentile(double q)
+    {
+        if (_count == 0) return 0;
+        Array.Copy(_samples, _scratch, _count);
+        Array.Sort(_scratch, 0, _count);
+        int rank = (int)Math.Ceiling(q * _count);   // 1-based nearest rank
+        if (rank < 1) rank = 1;
+        if (rank > _count) rank = _count;
+        return _scratch[rank - 1];
+    }
+
+    public long Max()
+    {
+        long max = 0;
+        for (int i = 0; i < _count; i++)
+            if (_samples[i] > max) max = _samples[i];
+        return max;
+    }
+}