feat(stability): Módulo 3 — Curva GZ + criterios IMO IS Code 2008

- gz_integrator.py: GZCurve, GZPoint, compute_gz_wall_sided (fórmula
  pared lateral), compute_gz_direct (integración Sutherland-Hodgman)
- imo_is2008.py: IMOCriterion, IMOResult, check_imo_is2008 —
  6 criterios A.2.1.1–A.2.1.6 del IS Code 2008 Cap.2
- gz_curve_widget.py: GZCurveWidget QPainter — curva cian, áreas
  sombreadas, líneas IMO, marcador AVS, tabla PASS/FAIL integrada
- main_window.py: GZCurveWidget en MOD_STABILITY, _compute_and_show_gz,
  _on_show_stability conectado al ribbon
- dark.qss: estilos GZCurveWidget
- test_module3_stability.py: 33 tests S-01..S-28 (315 total, todos pasan)

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

arshipdesign/stability/__init__.py

@@ -1 +1,31 @@
-# arshipdesign/stability
+"""
+arshipdesign.stability — estabilidad estática e intacta.
+
+Exporta:
+  - GZPoint, GZCurve          — estructuras de datos
+  - compute_gz_wall_sided      — fórmula de pared lateral (rápida)
+  - compute_gz_direct          — integración directa 3-D (precisa)
+  - IMOCriterion, IMOResult    — criterios IMO IS Code 2008
+  - check_imo_is2008           — verificación IS Code 2008 Cap.2
+"""
+from arshipdesign.stability.gz_integrator import (
+    GZPoint,
+    GZCurve,
+    compute_gz_wall_sided,
+    compute_gz_direct,
+)
+from arshipdesign.stability.imo_is2008 import (
+    IMOCriterion,
+    IMOResult,
+    check_imo_is2008,
+)
+
+__all__ = [
+    "GZPoint",
+    "GZCurve",
+    "compute_gz_wall_sided",
+    "compute_gz_direct",
+    "IMOCriterion",
+    "IMOResult",
+    "check_imo_is2008",
+]