capture: name the render source in the exact-bounds refusal, and put its one-frame tolerance under test

The tolerance is unchanged and now derived, not assumed: frameCountFor lands in {floor(L), ceil(L)}, so a non-frame-aligned window can never miss by more than a frame. Naming the source is what tells a self-bounding render from a short one.

src/core/capture/render_settings.cpp

@@ -100,6 +100,18 @@ RenderSettingsChoice renderSettingsFor(SourceMode mode, double /*wetDry*/) {
     return c;
 }
 
+const char* renderSourceLabel(SourceMode mode) {
+    switch (mode) {
+        case SourceMode::MasterMix:      return "master mix";
+        case SourceMode::TimeSelection:  return "master mix (time selection)";
+        case SourceMode::SelectedTracks: return "selected tracks via master";
+        case SourceMode::SelectedItems:  return "selected media items";
+        case SourceMode::RazorArea:      return "razor edits";
+        case SourceMode::Realtime:       return "realtime record";
+    }
+    return "unknown";  // unreachable for a valid enum; never claim a source
+}
+
 SourceMode sourceModeForScope(CaptureScope scope, bool itemExtentIsWindow) {
     switch (scope) {
         case CaptureScope::Item: