feat(python): expose vessel_cog/cog and update documentation

docs/source/api/python/hydrostatics.rst

@@ -152,7 +152,12 @@ HydrostaticState
    .. py:attribute:: cog
       :type: tuple[float, float, float] or None
 
-      Center of Gravity (LCG, TCG, VCG) vector, if provided.
+      Center of Gravity (LCG, TCG, VCG) vector, if provided. Matches ``displacement``.
+
+   .. py:attribute:: vessel_cog
+      :type: tuple[float, float, float] or None
+
+      Vessel Center of Gravity (LCG, TCG, VCG) vector (Ship-only). Matches ``vessel_displacement``.
 
    **Waterplane Properties**
 

docs/source/api/python/stability.rst

@@ -142,6 +142,8 @@ StabilityCurve
       - ``trim``: Trim angle in degrees
       - ``gz``: GZ value (m) (alias value)
       - ``value``: GZ or KN value (m)
+      - ``cog``: Total physical Center of Gravity (LCG, TCG, VCG)
+      - ``vessel_cog``: Vessel Center of Gravity (LCG, TCG, VCG)
       - ``is_flooding``: True if any opening is submerged
       - ``flooded_openings``: List of flooded opening names