[Feat] (Backend, MGPipe): carry the six per-axis compute limits in DynamicBackendParameters so MGPCaps has every backend-owned indexed answer, and pin them against glGetIntegeri_v on both backends

- P-1: MGPCaps is DynamicBackendParameters by inclusion (plan B section 4.4.1), but that struct carried MaxComputeWorkGroupInvocations and no per-axis GL_MAX_COMPUTE_WORK_GROUP_COUNT / GL_MAX_COMPUTE_WORK_GROUP_SIZE - the six numbers that ARE the backend-owned indexed answers surviving the getter retirement (GL_Getter.cpp and CompileEnv.cpp ask GLFunctionsTable::GetIntegeri_v for exactly these, DirectVulkan answers them from VkPhysicalDeviceLimits), so the interface had a hole where its only genuine indexed carrier should be. DynamicBackendParameters now has MaxComputeWorkGroupCount[3] / MaxComputeWorkGroupSize[3] with the GL 4.3 minimums as the no-backend defaults; DirectGLES fills them from glGetIntegeri_v inside the loader's bracketed probe run (GLESCapabilities carries them, logged with the other limits) and DirectVulkan from maxComputeWorkGroupCount / maxComputeWorkGroupSize through the loader's SaturateToInt like every other limit. Raw driver answers, as the invocations limit is: the frontend floors them at the shared MIN_COMPUTE_WORK_GROUP_* minimums itself.
- The GetIntegeri_v table path is untouched, as is GL_Getter and CompileEnv behaviour: retiring the getter in favour of the caps is P0.5, and this only makes sure the caps have what P0.5 needs.
- PipeCalls.def's footer no longer claims that "only GL_COMPUTE_WORK_GROUP_SIZE is a real backend answer and it lives in MGPCaps": the six limits live in MGPCaps, and GL_COMPUTE_WORK_GROUP_SIZE is a frontend link artifact (ProgramObject::GetComputeLocalSize, what GL_Program.cpp answers from), which AdvertisedLimitsScenario.ComputeLocalSizeComesFromTheLinkedProgram already pins. The MGPCaps size assertion is a composition of sizeof(DynamicBackendParameters) and follows the struct.
- AdvertisedLimitsScenario.ComputeWorkGroupLimitsAreTheCapsBlocksAnswer pins, on both lanes: answerability, the GL 4.3 floors, vector/indexed agreement, INVALID_VALUE past axis 2, and - through the new Harness/BackendCapsPeek translation unit, which is the one place the module looks past the GL API - that max(caps, minimum) equals the live glGetIntegeri_v answer axis by axis. Shown live by halving each backend's caps copy: both lanes fail with "MGPCaps carries 512 but glGetIntegeri_v answers 1024". On Android the module links the shipping .so (hidden visibility), so the peek returns false there and only the GL-visible half runs. ComputeWorkGroupCapabilities.TakesEveryAxisFromTheIndexedQuery in BackendLoaderTest pins the DirectGLES loader half against the fake driver, per axis and above the initialisers.
- Verified: AdvertisedLimitsScenario 20/20 on DirectGLES and DirectVulkan (llvmpipe / lavapipe), BackendLoaderTest green.

MobileGL/MG_Backend/DirectVulkan/DirectVulkan.cpp

@@ -674,7 +674,10 @@ namespace MobileGL::MG_Backend::DirectVulkan {
 
     // The two compute limits are the only indexed pnames a backend genuinely owns: they come
     // from the physical device, and MG_Impl/GLImpl/Getter/GL_Getter.cpp asks for them here so it
-    // can raise the answer to the GL required minimum. Every other indexed pname names FRONTEND
+    // can raise the answer to the GL required minimum. The same six numbers are carried in
+    // DynamicBackendParameters::MaxComputeWorkGroupCount/Size (filled at capability init from
+    // the same limits), which is their MGPCaps carrier once this entry retires - the
+    // AdvertisedLimitsScenario pins the two against each other. Every other indexed pname names FRONTEND
     // state (the indexed buffer bindings, the per-unit texture/sampler bindings, the image-unit
     // bindings, the viewport rectangles, the indexed capabilities) and is answered there before
     // the table is consulted, so the arms this function used to carry for