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/ui/themes/dark.qss

@@ -514,3 +514,29 @@ QLabel#hydroPlaceholder {
     background-color: #1a1d30;
     padding: 40px;
 }
+
+/* ─── Módulo 3: Estabilidad ─── */
+GZCurveWidget {
+    background: #0A0E1A;
+    border: 1px solid #1A2540;
+    border-radius: 4px;
+}
+#gzChartTitle {
+    color: #00FFCC;
+    font-family: "Rajdhani", "Segoe UI", sans-serif;
+    font-size: 11px;
+    font-weight: 600;
+    letter-spacing: 1px;
+}
+#gzPassLabel {
+    color: #00FF88;
+    font-family: "Consolas", "Courier New", monospace;
+    font-size: 11px;
+    font-weight: bold;
+}
+#gzFailLabel {
+    color: #FF4444;
+    font-family: "Consolas", "Courier New", monospace;
+    font-size: 11px;
+    font-weight: bold;
+}