feat(pipeline): MP0 - FrameProfiler facade (CPU/GPU/alloc/stages, 5s report)

Permanent frame profiler: FrameBoundary() at the top of OnRender measures
CPU frame time (swap-to-swap delta), brackets the frame in a GpuFrameTimer
TimeElapsed query (self-disabled under ACDREAM_WB_DIAG=1), and samples
per-frame allocated bytes + GC collection deltas. BeginStage(FrameStage)
scopes attribute CPU time to Update/Upload/ImGui. Emits one [frame-prof]
report line every ~5s while RenderingDiagnostics.FrameProfEnabled is on;
zero-cost stage scopes when off. Report formatting is a pure static
method, unit-tested for invariant-culture formatting and the gpu=off
fallback text.

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

tests/AcDream.App.Tests/FrameProfilerReportTests.cs

@@ -0,0 +1,48 @@
+using AcDream.App.Diagnostics;
+using Xunit;
+
+namespace AcDream.App.Tests;
+
+public class FrameProfilerReportTests
+{
+    [Fact]
+    public void FormatReport_IsInvariantAndComplete()
+    {
+        var cpu = new FrameStatsBuffer(16);
+        var gpu = new FrameStatsBuffer(16);
+        var alloc = new FrameStatsBuffer(16);
+        var stages = new[] { new FrameStatsBuffer(16), new FrameStatsBuffer(16), new FrameStatsBuffer(16) };
+        for (long i = 1; i <= 10; i++)
+        {
+            cpu.Push(i * 1000);      // 1..10 ms in µs
+            gpu.Push(i * 100);
+            alloc.Push(i * 1024);    // bytes
+            stages[0].Push(i * 200);
+            stages[1].Push(i * 50);
+            stages[2].Push(i * 10);
+        }
+
+        string line = FrameProfiler.FormatReport(
+            frameCount: 10, cpu: cpu, gpu: gpu, gpuActive: true,
+            alloc: alloc, gc0: 3, gc1: 1, gc2: 0, stages: stages);
+
+        Assert.StartsWith("[frame-prof]", line);
+        Assert.Contains("n=10", line);
+        Assert.Contains("cpu_ms p50=5.0 p95=10.0 p99=10.0 max=10.0", line);
+        Assert.Contains("gpu_ms p50=0.5", line);
+        Assert.Contains("alloc_kb p50=5.0 max=10.0", line);
+        Assert.Contains("gc=3/1/0", line);
+        Assert.Contains("upd p50=1.0", line);   // stage 0: 200µs·5 = 1.0 ms
+    }
+
+    [Fact]
+    public void FormatReport_GpuInactive_SaysWhy()
+    {
+        var empty = new FrameStatsBuffer(4);
+        string line = FrameProfiler.FormatReport(
+            frameCount: 0, cpu: empty, gpu: empty, gpuActive: false,
+            alloc: empty, gc0: 0, gc1: 0, gc2: 0,
+            stages: new[] { empty, empty, empty });
+        Assert.Contains("gpu=off(wbdiag)", line);
+    }
+}