fix(grips): prevent phantom snapping

Keep hidden pre-drag geometry out of snap candidates and avoid empty
scene invalidations. Require stable OTRACK dwell observations and cache
planar-solid visibility during live previews.

Refs #930

src/app/command_driver.rs

@@ -232,7 +232,6 @@ impl OpenCADStudio {
             self.tabs[i].dirty = dirty_before;
         }
 
-        self.grip_snap_wires.clear();
         self.grip_text_verts.clear();
         self.grip_text_slide = false;
         self.tabs[i].scene.clear_preview_wire();