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/capture_batch.cpp

@@ -214,6 +214,11 @@ void RunBatchCaptureItems(ReaSamplerSession& session)
             src.startSeconds = unit.startSeconds;
             src.endSeconds   = unit.endSeconds;
             src.sourceTracks.push_back(u.track);
+            // The unit's range IS this item's own extent (read above from
+            // D_POSITION/D_LENGTH) and it is the only selected item, so the
+            // selected-items render source prints exactly it — batch keeps the
+            // one-sample-per-item-at-item-extent semantics, unchanged.
+            src.itemExtentIsWindow = true;
             if (std::string g = guidString(u.track); !g.empty())
                 src.trackGuids.push_back(std::move(g));