capture: state the bounds tolerance as empirical, refuse unmeasurable renders, keep refused ones for diagnosis

The one-frame bound is not provable for a per-edge renderer; the test now shows where it breaks. Refused renders move out of the bank instead of being deleted, so the DAW experiment has something to read.

src/shell/capture/capture.h

@@ -87,7 +87,8 @@ enum class CaptureStatus {
     RenderFailed,        // the render action ran but produced no output file
     TransportBusy,       // realtime backend: transport already playing/recording — refused
     MultiTrackSelection, // a selected-tracks render over >1 track — would render N files
-    BoundsMismatch,      // the rendered file's frame count is not the requested window's
+    BoundsMismatch,      // the rendered file's frames are not the requested window's — or
+                         // could not be measured to say (render_bounds_gate)
 };
 
 struct CaptureResult {