[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_Pipe/MGPipeTypes.h

@@ -122,7 +122,10 @@ namespace MobileGL::MG_Pipe {
 
     struct MGPCaps {
         // The ~90 flat scalars the backends already publish, by inclusion rather than by
-        // restatement: a caps field added there must not need a second edit here.
+        // restatement: a caps field added there must not need a second edit here. This is
+        // also where the six per-axis compute limits (MaxComputeWorkGroupCount/Size) ride -
+        // the only indexed answers the device owns, and therefore the only ones that outlive
+        // the GetIntegeri_v table entry (see the PipeCalls.def footer).
         DynamicBackendParameters Dynamic;
         Uint64 CallMask; // MGPCapBit
         // The two halves that are not flat PODs travel as blobs: the format capability

MobileGL/MG_Pipe/PipeCalls.def

@@ -140,9 +140,18 @@
     X(SetSwapInterval,          MGPSwapInterval,            kCtxVerb,   kOptional)
 // clang-format on
 
-// Explicitly NOT migrated (plan 4.4.6 / appendix A "explicit deletions"): GetIntegeri_v,
-// GetInteger64i_v, GetProgramiv (only GL_COMPUTE_WORK_GROUP_SIZE is a real backend answer
-// and it lives in MGPCaps), ShaderStorageBlockBinding (folded into MGPProgramDesc's
-// reflection archive), set_pixel_unpack_state (no such state crosses the line - plan 4.6
-// D5), a compressed-format concept, pipe_transfer, and the stage dimension of
-// set_sampler_views (MobileGL's texture unit space is merged, not per stage - plan 4.4.3).
+// Explicitly NOT migrated (plan 4.4.6 / appendix A "explicit deletions"):
+//   - GetIntegeri_v / GetInteger64i_v. The six backend-owned answers they carry -
+//     GL_MAX_COMPUTE_WORK_GROUP_COUNT and GL_MAX_COMPUTE_WORK_GROUP_SIZE, three axes each,
+//     the only indexed pnames the device rather than the frontend answers - live in MGPCaps
+//     as DynamicBackendParameters::MaxComputeWorkGroupCount / MaxComputeWorkGroupSize, filled
+//     by both backends at capability init (DirectGLES from glGetIntegeri_v, DirectVulkan from
+//     VkPhysicalDeviceLimits) and floored by the frontend. Every other indexed pname names
+//     frontend state and is answered before any table is consulted.
+//   - GetProgramiv. GL_COMPUTE_WORK_GROUP_SIZE is a FRONTEND link artifact
+//     (ProgramObject::GetComputeLocalSize, what GL_Program.cpp has always answered from), not
+//     a backend answer at all; nothing a backend knows about a program crosses this way.
+//   - ShaderStorageBlockBinding (folded into MGPProgramDesc's reflection archive),
+//     set_pixel_unpack_state (no such state crosses the line - plan 4.6 D5), a
+//     compressed-format concept, pipe_transfer, and the stage dimension of set_sampler_views
+//     (MobileGL's texture unit space is merged, not per stage - plan 4.4.3).