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

src/core/capture/render_settings.cpp

@@ -112,11 +112,27 @@ SourceMode sourceModeForScope(CaptureScope scope, bool itemExtentIsWindow) {
     return SourceMode::SelectedTracks;
 }
 
-bool isMultiTrackRangedItemRender(CaptureScope scope, SourceMode mode,
-                                  int sourceTrackCount) {
-    return scope == CaptureScope::Item
-        && mode  == SourceMode::SelectedTracks
-        && sourceTrackCount > 1;
+bool isMultiTrackStemRender(SourceMode mode, int sourceTrackCount) {
+    return mode == SourceMode::SelectedTracks && sourceTrackCount > 1;
+}
+
+std::string multiTrackRefusalMessage(CaptureScope scope) {
+    switch (scope) {
+        case CaptureScope::Item:
+            return "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.";
+        case CaptureScope::Track:
+            return "A track capture renders the selected tracks through the master, "
+                   "and more than one track cannot land as a single file. Capture one "
+                   "track at a time, or route them into a folder/bus track and capture "
+                   "that (a folder's own output is its children summed).";
+    }
+    // Unreachable for a valid enum; a refusal with no way out is still better than a
+    // silent one, so fail closed to the scope-agnostic half of the message.
+    return "This selection spans more than one track, which cannot land as a single "
+           "file. Capture one track at a time.";
 }
 
 RangeSource inferRangeSource(bool hasRazorArea) {