[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_Test/SanityTest.cpp

@@ -907,3 +907,28 @@ TEST(RenderStateSanity, ColorMaskIndexedStoresAndReadsBack) {
 
     MG_State::pGLContext.reset();
 }
+
+TEST(RenderStateSanity, PrimitiveRestartIndexStoresAndReadsBack) {
+    using namespace MobileGL;
+    using namespace MobileGL::MG_Impl::GLImpl;
+    MG_State::pGLContext = MakeUnique<MG_State::GLState::GLContext>();
+
+    // Default is 0.
+    GLint value = -1;
+    GetIntegerv(GL_PRIMITIVE_RESTART_INDEX, &value);
+    EXPECT_EQ(value, 0);
+
+    // Any GLuint round-trips and generates no error.
+    PrimitiveRestartIndex(0xFFFFu);
+    EXPECT_EQ(GetError(), GL_NO_ERROR);
+    GetIntegerv(GL_PRIMITIVE_RESTART_INDEX, &value);
+    EXPECT_EQ(value, 0xFFFF);
+
+    // The full 32-bit range round-trips (read back as the same bit pattern).
+    PrimitiveRestartIndex(0xFFFFFFFFu);
+    GetIntegerv(GL_PRIMITIVE_RESTART_INDEX, &value);
+    EXPECT_EQ(static_cast<GLuint>(value), 0xFFFFFFFFu);
+    EXPECT_EQ(GetError(), GL_NO_ERROR);
+
+    MG_State::pGLContext.reset();
+}