[Feat, Test] (MG_Backend/DirectVulkan, MG_Test): Magma advertisement + the parallel-compile test net

Same gated push as the Espryt commit. The tests ride here because they
exercise both backends' advertisement paths and every piece of the
extension: ParallelShaderCompileTest (13 unit cases - the held-job proof
that GL_FALSE is observable and a second poll still shows outstanding work,
program equivalent, untouched objects read TRUE, always-TRUE with async
off, unknown pnames still INVALID_ENUM, zero-count join+inline for compiles
AND links, nonzero restores while Initialize() does not, clamping and
0xFFFFFFFF and KHR/ARB sharing one state, the getter vs the budget, the
string tracking configuration through glGetString AND glGetStringi) and
AsyncCompileScenario (5 real-GPU cases per the design: 64-compile polling,
forced-join correctness, string/thread-count checks against a live driver,
zero-count synchronous settlement, and async-vs-sync frames rendered
byte-identical with quadrant signatures so two identically-wrong images
cannot pass). Verified 5/5 on NVIDIA in the full 2x2 backend x flag matrix
with MOBILEGL_ITEST_REQUIRE_GPU=1.

MobileGL/MG_IntegrationTest/CMakeLists.txt

@@ -50,6 +50,7 @@ add_executable(MobileGLIntegrationTest
     Scenarios/CrossFrameBufferScenario.cpp
     Scenarios/ResidentIndexScenario.cpp
     Scenarios/MultiDrawScenario.cpp
+    Scenarios/AsyncCompileScenario.cpp
 )
 
 target_include_directories(MobileGLIntegrationTest PRIVATE