capture: refuse the multi-track ranged item render, silence the track's children and receives for it, and gate every exact-bounds capture on its frame count

src/shell/capture/capture.h

@@ -71,6 +71,8 @@ enum class CaptureStatus {
     UnsupportedFormat,  // requested bit depth has no known REAPER blob (Float32 only)
     RenderFailed,       // the render action ran but produced no output file
     TransportBusy,      // realtime backend: transport already playing/recording — refused
+    MultiTrackRange,    // a ranged item capture whose items span >1 track — would render N files
+    BoundsMismatch,     // the rendered file's frame count is not the requested window's
 };
 
 struct CaptureResult {