fix(ui): smooth B-spline section/waterline curves + 3D ready-signal sync

- viewer_3d.py: add `ready` Signal emitted once QtInteractor finishes init
- main_window.py: connect ready signal to sync active hull into 3D viewer on startup
- viewer_lines.py: add _smooth_pts helper; replace straight polylines in BodyPlanViewer
  and PlanViewer CAPA 3 with B-spline interpolated curves (80 sample points)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

arshipdesign/ui/main_window.py

@@ -841,6 +841,7 @@ class MainWindow(QMainWindow):
         from arshipdesign.ui.widgets.viewer_3d import Viewer3DWidget, _PYVISTA_OK
         if _PYVISTA_OK:
             self._viewer_3d = Viewer3DWidget()
+            self._viewer_3d.ready.connect(self._on_3d_viewer_ready)
             vp = self._module_area.four_viewport.viewport("perspective")
             if vp is not None:
                 vp.set_canvas(self._viewer_3d)
@@ -1357,6 +1358,15 @@ class MainWindow(QMainWindow):
         self._load_hull_viewers(hull, _skip_offsets_editor=True)
         self.statusBar().showMessage(f"Offsets actualizados — {hull.name}")
 
+    def _on_3d_viewer_ready(self) -> None:
+        """El plotter 3D terminó de inicializarse — sincronizar con el casco activo."""
+        if self._current_hull is not None and self._viewer_3d is not None:
+            try:
+                self._current_hull.invalidate()
+                self._viewer_3d.load_hull(self._current_hull)
+            except Exception as exc:
+                logger.warning("Error al sincronizar 3D tras init: %s", exc)
+
     def _on_offsets_edited_from_viewer(self, offsets_table) -> None:
         """Slot: fin del drag — persistir + actualizar 3D + hidrostáticos.