feat(topbar): the recording lives in the pill — green pulse, REC clock, Stop Recording

Build 730 (creator):

- Left pill while recording: solid #2ECC71 dot pulsing 1.0 <-> 0.2 on the existing one-second timer, and
  "REC: hh:mm:ss" in Consolas so the digits cannot jitter the pill's width. Idle face unchanged.
- Right pill while recording: a stop square and "Stop Recording"; clicking it runs the normal stop, which
  halts and resets the clock and returns the pair to "Local MP4" / "Start Recording".
- The separate reality line (dot + REC/LIVE + elapsed + End) is now LIVE-ONLY: a recording would otherwise
  print the same three facts twice. RunningVisible is IsLive, and ShowOutputStage no longer hides itself
  for a recording (it must be on screen to carry the clock and the stop).

Two rules collided and are now separated rather than merged: a creator's Stop on a recording restores the
Local MP4 pair (build 730), while a pipeline FAILURE still clears both pills (the 2026-09-01 zombie rule).
StopStream(afterFailure:) carries the difference, and the pump-failure path passes it.

RealAppHost.Run was rethrowing with `throw failure;`, which destroyed the original stack — a failing
assertion inside the action reported the wrapper's line instead of its own. ExceptionDispatchInfo now
preserves it; that is how the two failures above were located in one pass instead of five.

680 pass (one audio-pipe instance flake under full-suite load, passes alone), 0 warnings.
Scope justification: Assets/starting-background.jpg is outside this unit — the creator's replacement art,
still awaiting their visual check.

ytLive.Tests/RealAppCollection.cs

@@ -64,8 +64,10 @@ public sealed class RealAppHost : IDisposable
                 failure = ex;
             }
         });
+        // Rethrow preserving the ORIGINAL stack: `throw failure;` reset it to this line, which hid
+        // which assertion actually failed inside the passed action.
         if (failure != null)
-            throw failure;
+            System.Runtime.ExceptionServices.ExceptionDispatchInfo.Capture(failure).Throw();
     }
 
     /// <summary>Awaits <paramref name="action"/> on the App's STA thread and rethrows