chore(pump): per-stage timing stats — render vs submit, 5s windows

Take-two forensics: rawvideo carries no per-frame timestamps — ffmpeg stamps frames
by ARRIVAL at the declared fps, so a 2fps producer yields a 30x time-lapse, 0.6s-long
'60fps' file with zero errors anywhere (explains the creator's accelerated webcam
motion + truncated length observations). The pump now logs frames-produced-vs-target
per 5s with the avg render/submit split — take three names the slow stage instead
of us theorizing. FramePumpTests 9/9. (Multi-commit working unit: remaining declared
files land in the next commit.)

Services/Encoder/FramePump.cs

@@ -218,6 +218,30 @@ public sealed class FramePump : IDisposable
         var interval = TimeSpan.FromSeconds(1d / Math.Max(1, options.Fps));
         var lastTick = System.Diagnostics.Stopwatch.StartNew();
 
+        // Stage timing (2026-09-01, take two): rawvideo carries no per-frame
+        // timestamps — ffmpeg stamps frames by ARRIVAL at the declared fps. A producer
+        // slower than the declared rate yields a time-lapsed, short file (observed:
+        // 39 frames in 27 wall-seconds ≈ 30x at 60fps) with no error anywhere.
+        // Log the render/submit split every 5s so the next take names the stage.
+        var renderSw = new System.Diagnostics.Stopwatch();
+        var submitSw = new System.Diagnostics.Stopwatch();
+        long renderTicks = 0, submitTicks = 0;
+        int statFrames = 0;
+        var statsNext = DateTime.UtcNow + TimeSpan.FromSeconds(5);
+        void ReportStats()
+        {
+            if (DateTime.UtcNow < statsNext) return;
+            var target = 5d / interval.TotalSeconds; // frames expected per window
+            _log?.Invoke(statFrames == 0
+                ? "FramePump stats: NO frames produced in 5s (loop stalled?)"
+                : $"FramePump stats: {statFrames}/{target:F0} frames per 5s, " +
+                  $"avg render {renderTicks / (double)System.Diagnostics.Stopwatch.Frequency * 1000 / statFrames:F1}ms, " +
+                  $"avg submit {submitTicks / (double)System.Diagnostics.Stopwatch.Frequency * 1000 / statFrames:F1}ms");
+            renderTicks = submitTicks = 0;
+            statFrames = 0;
+            statsNext = DateTime.UtcNow + TimeSpan.FromSeconds(5);
+        }
+
         try
         {
             while (!ct.IsCancellationRequested)
@@ -239,6 +263,7 @@ public sealed class FramePump : IDisposable
                     }
 
                     VideoFrame frame;
+                    renderSw.Restart();
                     if (_transition is { Active: true } transition && _fromSceneProvider != null)
                     {
                         var fromScene = _fromSceneProvider();
@@ -255,11 +280,18 @@ public sealed class FramePump : IDisposable
                         frame = RenderScene(scene, compositorOptions, socialBarFrame, socialBarTop);
                         lastTick.Restart();
                     }
+                    renderSw.Stop();
+                    renderTicks += renderSw.ElapsedTicks;
 
                     IFfmpegEncoder? encoder;
                     lock (_gate) encoder = _encoder;
                     if (encoder == null) break;
+                    submitSw.Restart();
                     await encoder.SubmitFrameAsync(frame, ct);
+                    submitSw.Stop();
+                    submitTicks += submitSw.ElapsedTicks;
+                    statFrames++;
+                    ReportStats();
                 }
                 await _pacingDelay(interval, ct);
             }