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

src/shell/capture/capture_orchestrator.cpp

@@ -184,17 +184,14 @@ CaptureResult renderOffline(CaptureScope scope,
 {
     // Refused BEFORE anything is touched, so the refusal path has nothing to
     // restore. This is the seam BOTH a fresh capture and a recipe replay cross, so
-    // neither can land the multi-stem render the predicate names.
-    if (isMultiTrackRangedItemRender(scope, req.sourceMode,
-                                     static_cast<int>(sourceTracks.size())))
+    // neither can land the multi-stem render the predicate names — and both scopes
+    // reach it, so a track capture and a ranged item capture refuse alike.
+    if (isMultiTrackStemRender(req.sourceMode,
+                               static_cast<int>(sourceTracks.size())))
     {
         CaptureResult refused;
-        refused.status  = CaptureStatus::MultiTrackRange;
-        refused.message =
-            "This range is narrower than the selected items, so it renders through "
-            "their tracks -- and those items span more than one track, which this "
-            "shape cannot land as a single file. Capture one track's items at a "
-            "time, or make the range match the items' extent.";
+        refused.status  = CaptureStatus::MultiTrackSelection;
+        refused.message = multiTrackRefusalMessage(scope);
         return refused;
     }