FramePumpTests: thread-safe log sink — the full-suite failure was a test race

The suite failed Stats_ReportRealRendersAndDistinctWebcamFrames_NotJustPaddedSubmits
with "Collection was modified; enumeration operation may not execute" at its OWN
log.Any(...) line (FramePumpTests.cs:606): the test handed FramePump a plain
List<string> and enumerated it from the test thread while the pump reported stats
from its loop thread — pacingDelay is Task.Yield(), so the loop really is concurrent.
Production passes the thread-safe AppLog, so the app was never involved.

It passed 3/3 in isolation, which is exactly how a data race presents as a
load-sensitive flake. The trigger: the previous commit ("Per-element composite timing
+ scene dump") added log lines emitted FROM the pump thread, raising the collision
rate from rare to routine — a latent race in the old test, promoted by the new
traffic.

Fix: ConcurrentBag<string> (enumeration is a moment-in-time snapshot) for the sink,
2 call sites + the NewPump parameter. No app code touched.

Proof: full suite 428/428 passed, 0 warnings (the run that surfaced the failure was
before the fix; the same failure has not recurred).

Docs in the same commit: MyMistakes.md — "A 'known failure' label without a recorded
cause = a bug on life support" gained this second instance, with the rule that a
test failing ONLY under load means a shared mutable collection on the producer's
thread, not a retry.

ytLive.Tests/FramePumpTests.cs

@@ -1,3 +1,4 @@
+using System.Collections.Concurrent;
 using System.Net;
 using System.Text;
 using System.Text.RegularExpressions;
@@ -74,9 +75,12 @@ public class FramePumpTests
         return scene;
     }
 
+    // The log sink must be thread-safe: FramePump reports stats from its OWN loop thread while
+    // the test enumerates this collection (a plain List<string> threw "Collection was modified"
+    // under full-suite load — the pump's extra parts/scene-dump lines made it fire far more often).
     private static FramePump NewPump(FakeEncoder encoder, Func<EncoderOptions?>? options = null,
         Func<Scene?>? scene = null, Func<SceneElement, VideoFrame?>? resolve = null,
-        List<string>? log = null,
+        ConcurrentBag<string>? log = null,
         Func<(VideoFrame? Frame, SocialBarPosition Position)>? socialBar = null)
     {
         return new FramePump(
@@ -197,7 +201,7 @@ public class FramePumpTests
     public async Task Start_WithoutRtmpUrl_SkipsEncoder()
     {
         var encoder = new FakeEncoder();
-        var log = new List<string>();
+        var log = new ConcurrentBag<string>();
         using var pump = NewPump(encoder, options: () => null, log: log);
 
         await pump.StartAsync();
@@ -589,7 +593,7 @@ public class FramePumpTests
         var scene = new Scene { Name = "Live" };
         scene.Elements.Add(new WebcamSceneConfig { Name = "Webcam", IsVisible = true });
         var encoder = new FakeEncoder();
-        var log = new List<string>();
+        var log = new ConcurrentBag<string>();
         var epoch = 0L;
         using var pump = NewPump(encoder, scene: () => scene, log: log, resolve: e =>
         {