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/shell/capture/scope_resolve.h

@@ -34,6 +34,12 @@ struct ResolvedSource
     double                    endSeconds   = 0.0;
     std::vector<MediaTrack*>  sourceTracks;
     std::vector<std::string>  trackGuids;
+
+    // Item scope only: does the selected items' own extent already print
+    // [startSeconds, endSeconds)? Feeds sourceModeForScope. Defaults false so a
+    // hand-built source fails closed to the time-bounded render — a caller whose
+    // range IS the item extent (batch item capture) says so explicitly.
+    bool                      itemExtentIsWindow = false;
 };
 
 // Reads every track's P_RAZOREDITS and returns the union of parsed track-audio