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/render_selection.h

@@ -0,0 +1,34 @@
+#pragma once
+// The transient DAW track selection a selected-tracks render requires. REAPER's
+// &128 source prints whatever tracks are selected, not the tracks the request
+// names, so the render owns the selection for its duration and hands it back.
+// The .cpp includes reaper_plugin_functions.h WITHOUT REAPERAPI_IMPLEMENT.
+
+#include <vector>
+
+#include "shell/capture/capture.h"  // MediaTrack fwd
+
+namespace reasampler::capture {
+
+// RAII: selects exactly `tracks`, restores the project's original track selection
+// on every exit path (non-destructive — selection flags only, no restructuring).
+// Two callers need this and neither has the right selection standing: a ranged
+// item capture renders through the items' tracks while the user's track selection
+// is unrelated, and re-capture-from-source resolves its tracks by GUID and selects
+// nothing at all. An empty `tracks` is a deliberate no-op — forcing an empty
+// selection would render silence.
+class RenderTrackSelection
+{
+public:
+    explicit RenderTrackSelection(const std::vector<MediaTrack*>& tracks);
+    ~RenderTrackSelection();
+
+    RenderTrackSelection(const RenderTrackSelection&)            = delete;
+    RenderTrackSelection& operator=(const RenderTrackSelection&) = delete;
+
+private:
+    std::vector<MediaTrack*> original_;
+    bool                     applied_ = false;
+};
+
+}  // namespace reasampler::capture