fix(ui): 3D stale cache + node/waterline visual hierarchy

hull.py
  - add invalidate() — clears _surface NURBS cache on in-place
    offsets edit; fixes 3D viewer showing old geometry after drag

main_window.py
  - call hull.invalidate() before load_hull() in
    _on_offsets_edited_from_viewer so PyVista always rebuilds mesh
    from the updated offsets

viewer_lines.py
  - 4-layer drawing order: grid → control-net → hull-curves → nodes
  - nodes changed from 4px white-blue circles to 6px orange squares
    (_NODE_NORMAL #FF8000) — unambiguous visual language vs blue/green
    hull curves
  - _draw_cnet_bodyplan / _draw_cnet_planview helpers: thin muted
    control-net mesh (transverse + longitudinal edges) drawn between
    grid and bold hull curves, matching Maxsurf/DelftShip visual style
  - waterline reference lines made more muted (_GRID_WL dotted)
  - all old _GRID / _CPT_* references replaced with new palette

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

arshipdesign/core/hull.py

@@ -98,6 +98,16 @@ class Hull:
             self._surface = self._build_surface()
         return self._surface
 
+    def invalidate(self) -> None:
+        """Invalida la caché de la superficie NURBS.
+
+        Llamar siempre que se modifiquen los offsets in-place
+        (p.ej. arrastre interactivo en los visores 2D) para que la
+        próxima llamada a ``surface`` o ``to_mesh`` reconstruya la
+        geometría desde los datos actualizados.
+        """
+        self._surface = None
+
     def _build_surface(self) -> LoftedSurface:
         sections_data = []
         u_arr = self.offsets.x_stations / self.lpp  # normalizar a [0,1]