Fix feature highlight wiped by scene re-render

Selecting a feature drew the cyan overlay, but the concurrent scene re-render
(render_scene -> plotter.clear) wiped it, leaving only the yellow selected
board. _on_changed now re-applies the feature overlay after render_scene, so
the cyan highlight (and red edit preview) persist.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/woodshop/gui/main_window.py

@@ -126,6 +126,8 @@ class MainWindow(QMainWindow):
     def _on_changed(self):
         scene = self.controller.scene
         self.viewport.render_scene(scene, self.controller.selected)
+        # render_scene clears all actors — re-apply the feature overlay on top.
+        self.viewport.set_preview(self.controller.preview, self.controller.preview_kind)
         bf = sum(board_feet(p.stock, p.length_in) for p in scene.parts)
         sel = self.controller.selected
         sel_txt = (f"{len(sel)} selected" if len(sel) > 1