[Feat] (MG_Impl/GLImpl, MG_State): implement glPrimitiveRestartIndex

Store the primitive restart index as render state and report it through
glGetIntegerv(GL_PRIMITIVE_RESTART_INDEX), replacing the stub and the
hardcoded 0 in the getter.

- RenderState gains a PrimitiveRestartIndex field (default 0) with
  set/get accessors and GLContext wrappers.
- glPrimitiveRestartIndex accepts any GLuint and generates no error.
- glGetIntegerv(GL_PRIMITIVE_RESTART_INDEX) now reads the stored value.

This is the state layer only. The backends do not yet honor an arbitrary
restart index at draw time -- Vulkan and GLES support only the fixed
all-ones restart value (GL_PRIMITIVE_RESTART_FIXED_INDEX) -- so a non-
default index is tracked and queryable but not yet applied to indexed
draws.

Tests: RenderStateSanity round-trip (default 0, mid value, and the full
32-bit range). Full SanityTest sweep green (31/31).

MobileGL/MG_State/GLState/Core.cpp

@@ -372,6 +372,14 @@ namespace MobileGL::MG_State {
             return m_renderState.GetPolygonModeBack();
         }
 
+        void GLContext::SetPrimitiveRestartIndex(Uint32 index) {
+            m_renderState.SetPrimitiveRestartIndex(index);
+        }
+
+        Uint32 GLContext::GetPrimitiveRestartIndex() const {
+            return m_renderState.GetPrimitiveRestartIndex();
+        }
+
         void GLContext::SetPointSize(Float size) {
             m_renderState.SetPointSize(size);
         }

MobileGL/MG_State/GLState/Core.h

@@ -136,6 +136,8 @@ namespace MobileGL {
                 void SetPolygonMode(GLenum front, GLenum back);
                 GLenum GetPolygonModeFront() const;
                 GLenum GetPolygonModeBack() const;
+                void SetPrimitiveRestartIndex(Uint32 index);
+                Uint32 GetPrimitiveRestartIndex() const;
                 void SetCapability(CapabilityInput cap, Bool enabled);
                 Bool IsCapabilityEnabled(CapabilityInput cap) const;
                 void SetCapabilityIndexed(CapabilityInput cap, Uint index, Bool enabled);

MobileGL/MG_State/GLState/RenderState/RenderState.cpp

@@ -133,6 +133,16 @@ namespace MobileGL {
                 return m_parameters.PolygonModeBack;
             }
 
+            void RenderState::SetPrimitiveRestartIndex(Uint32 index) {
+                if (m_parameters.PrimitiveRestartIndex == index) return;
+                m_parameters.PrimitiveRestartIndex = index;
+                ++m_version;
+            }
+
+            Uint32 RenderState::GetPrimitiveRestartIndex() const {
+                return m_parameters.PrimitiveRestartIndex;
+            }
+
             void RenderState::SetPointSize(Float size) {
                 if (m_parameters.PointSize == size) return;
 

MobileGL/MG_State/GLState/RenderState/RenderState.h

@@ -269,6 +269,10 @@ namespace MobileGL {
         GLenum PolygonModeFront = GL_FILL;
         GLenum PolygonModeBack = GL_FILL;
 
+        // Primitive restart index (glPrimitiveRestartIndex); consumed when GL_PRIMITIVE_RESTART is
+        // enabled during an indexed draw. Default 0.
+        Uint32 PrimitiveRestartIndex = 0;
+
         // Scissor
         Bool ColorLogicOpEnabled = false;
         Bool DebugOutputEnabled = false;
@@ -326,6 +330,8 @@ namespace MobileGL {
                 void SetPolygonMode(GLenum front, GLenum back);
                 GLenum GetPolygonModeFront() const;
                 GLenum GetPolygonModeBack() const;
+                void SetPrimitiveRestartIndex(Uint32 index);
+                Uint32 GetPrimitiveRestartIndex() const;
 
                 // Capabilities
                 void SetCapability(CapabilityInput cap, Bool enabled);