Refactor monolithic VSG GUI implementation

Refactor monolithic VSG GUI implementation into focused modules:
- vsg_config.h: centralized compile-time constants
- vsg_materials.h/cpp: PBR material factory functions
- vsg_lighting.h/cpp: multi-light studio setup
- vsg_water_surface.h/cpp: animated/static water surface rendering
- vsg_gui_component.h/cpp: ImGui overlay components

Add runtime Viewer Settings panel (bottom-left) with controls:
- Water surface visibility toggle
- Grid resolution selector (32/64/96/128)
- Update rate cap (10-60 Hz)
- Optional status line display

Settings are visualization-only (no physics changes) and take
effect immediately without recompilation. Defaults match previous behavior for backward compatibility.

Author: dav-og

CMakeLists.txt

@@ -418,6 +418,17 @@ endif()
 if(HYDROCHRONO_ENABLE_VSG)
   set(HCGUI_SOURCES ${HCGUI_SOURCES}
       src/gui/guihelperVSG.cpp
+      src/gui/vsg_gui_component.cpp
+      src/gui/vsg_lighting.cpp
+      src/gui/vsg_materials.cpp
+      src/gui/vsg_water_surface.cpp
+  )
+  set(HCGUI_HEADERS ${HCGUI_HEADERS}
+      src/gui/vsg_config.h
+      src/gui/vsg_gui_component.h
+      src/gui/vsg_lighting.h
+      src/gui/vsg_materials.h
+      src/gui/vsg_water_surface.h
   )
 endif()