capture: render a ranged item capture time-bounded — the selected-items source can't narrow a window, only a full-extent one uses it

sourceModeForScope now takes the item extent vs. the requested window. Full-extent item captures and the batch keep the old path unchanged.

src/core/capture/render_settings.cpp

@@ -100,12 +100,16 @@ RenderSettingsChoice renderSettingsFor(SourceMode mode, double /*wetDry*/) {
     return c;
 }
 
-SourceMode sourceModeForScope(CaptureScope scope) {
+SourceMode sourceModeForScope(CaptureScope scope, bool itemExtentIsWindow) {
     switch (scope) {
-        case CaptureScope::Item:   return SourceMode::SelectedItems;
+        case CaptureScope::Item:
+            return itemExtentIsWindow ? SourceMode::SelectedItems
+                                      : SourceMode::SelectedTracks;
         case CaptureScope::Track:  return SourceMode::SelectedTracks;
     }
-    return SourceMode::SelectedItems;  // unreachable for a valid enum; fail closed
+    // Unreachable for a valid enum; fail closed to the time-bounded render, which
+    // honors the requested bounds whatever the selection is.
+    return SourceMode::SelectedTracks;
 }
 
 RangeSource inferRangeSource(bool hasRazorArea) {