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.
 

arshipdesign/ui/widgets/viewer_3d.py

@@ -17,7 +17,7 @@ import logging
 from typing import Optional
 
 import numpy as np
-from PySide6.QtCore import Qt, QTimer
+from PySide6.QtCore import Qt, QTimer, Signal
 from PySide6.QtWidgets import QLabel, QVBoxLayout, QWidget
 
 logger = logging.getLogger("ui.viewer_3d")
@@ -50,6 +50,8 @@ class Viewer3DWidget(QWidget):
     >>> v.reset_camera()
     """
 
+    ready: Signal = Signal()
+
     def __init__(self, parent: Optional[QWidget] = None) -> None:
         super().__init__(parent)
         self._plotter: Optional["QtInteractor"] = None
@@ -114,6 +116,7 @@ class Viewer3DWidget(QWidget):
             else:
                 self._load_default_wigley()
             self._ready = True
+            self.ready.emit()
             logger.info("Viewer3DWidget: QtInteractor iniciado correctamente")
 
         except Exception as exc:

arshipdesign/ui/widgets/viewer_lines.py

@@ -365,6 +365,25 @@ def _draw_cnet_planview(p: QPainter, ot, w2s_fn) -> None:
         p.drawPath(path)
 
 
+def _smooth_pts(pts_2d: np.ndarray, n: int = 60) -> np.ndarray:
+    """Muestrea n puntos de una B-spline interpolada a través de pts_2d.
+
+    pts_2d : shape (m, 2)
+    Returns shape (n, 2).  Si hay < 4 puntos o falla el spline,
+    devuelve los puntos originales sin modificar.
+    """
+    from arshipdesign.geometry.nurbs_curve import BSplineCurve
+    m = len(pts_2d)
+    if m < 4:
+        return pts_2d
+    try:
+        k = min(3, m - 1)
+        curve = BSplineCurve(pts_2d, degree=k)
+        return curve.sample(n)        # shape (n, 2)
+    except Exception:
+        return pts_2d
+
+
 # ─────────────────────────────────────────────────────────────────────────────
 # 1. Body Plan — secciones transversales
 # ─────────────────────────────────────────────────────────────────────────────
@@ -475,14 +494,20 @@ class BodyPlanViewer(_BaseViewer):
             z_arr = ot.z_waterlines
             sign  = 1.0 if is_fwd else -1.0
 
+            # Smooth B-spline through the section data points
+            raw = np.column_stack([y_arr * sign, z_arr])
+            # Close to keel: append (0, 0) as the last interpolation point
+            keel_row = np.array([[0.0, 0.0]])
+            raw_closed = np.vstack([raw, keel_row])
+            smooth = _smooth_pts(raw_closed, n=80)
+
             path = QPainterPath()
-            for k, (y, z) in enumerate(zip(y_arr, z_arr)):
-                pt = self._w2s(sign * y, z)
-                if k == 0:
+            for k_pt in range(len(smooth)):
+                pt = self._w2s(smooth[k_pt, 0], smooth[k_pt, 1])
+                if k_pt == 0:
                     path.moveTo(pt)
                 else:
                     path.lineTo(pt)
-            path.lineTo(self._w2s(0.0, 0.0))
             p.drawPath(path)
 
         # Flotación de diseño (encima de todo lo anterior)
@@ -691,15 +716,20 @@ class PlanViewer(_BaseViewer):
                 width = 2.2
             else:
                 color = QColor(_WATERLINE)
-                color.setAlphaF(0.40 + 0.50 * frac)
-                width = 1.1
+                color.setAlphaF(0.45 + 0.45 * frac)
+                width = 1.2
 
             p.setPen(QPen(color, width))
             p.setBrush(Qt.BrushStyle.NoBrush)
+
+            # Smooth B-spline through the waterline data points
+            raw = np.column_stack([ot.x_stations, ot.data[:, j]])
+            smooth = _smooth_pts(raw, n=80)
+
             path = QPainterPath()
-            for i, (x, y) in enumerate(zip(ot.x_stations, ot.data[:, j])):
-                pt = self._w2s(x, y)
-                if i == 0:
+            for k_pt in range(len(smooth)):
+                pt = self._w2s(smooth[k_pt, 0], smooth[k_pt, 1])
+                if k_pt == 0:
                     path.moveTo(pt)
                 else:
                     path.lineTo(pt)