Refuse every multi-track selected-tracks render, both scopes; make a failed mono collapse observable

src/shell/capture/capture_realtime_finalize.cpp

@@ -186,7 +186,7 @@ CaptureResult finalizeRecording(ReaProject* proj, MediaTrack* temp,
 
     // Channel-domain rewrite, after the frame-domain trim so it acts on the final
     // frame set; it preserves the frame count, so the trimmed length above still holds.
-    const bool collapsedToMono = collapseCapturedFileToMono(destPath);
+    const MonoCollapseOutcome collapseOutcome = collapseCapturedFileToMono(destPath);
 
     // Pure recorded-capture -> Sample mapping (identity, bounds echo, tier).
     RecordedCapture cap;
@@ -225,8 +225,7 @@ CaptureResult finalizeRecording(ReaProject* proj, MediaTrack* temp,
                      std::to_string(request.startSeconds) + "s, " +
                      std::to_string(request.endSeconds) + "s] (recorded " +
                      std::to_string(result.sample.lengthSeconds) + "s) -> " +
-                     paths.relativePath +
-                     (collapsedToMono ? " (collapsed to mono)" : "");
+                     paths.relativePath + monoCollapseSuffix(collapseOutcome);
     return result;
 }