[Refactor] (MG_State, MG_Util): join-by-construction link/compile artifacts (P1 stage 2)

Still fully synchronous - EnsureLinkJoined()/EnsureCompileJoined() are empty
inline no-ops (verified to fold away at every one of the ~1200 call sites;
this project builds without LTO) - but every read of link- or compile-produced
state now goes through a private accessor the compiler enforces, so when
stage 4 moves the bodies onto pool workers, 'which reads must join' is a
type-system fact instead of a 400-line audit.

- ProgramObject: the 31 fields ResetLinkArtifacts clears plus the 5 link
  outputs it forgot (infoLog, linkedFragData{Location,Index}, the geometry
  strip-capture pair) move into a nested LinkArtifacts behind Artifacts().
  ResetLinkArtifacts is now a worker-safe pure clear; the link-observable
  version bumps (backendState/link/uboContent) move to a GL-thread-only
  BumpLinkObservableVersions() called once from Link()'s prologue and from
  glProgramBinary's mandated failure - the link body never writes them, so
  a stage-4 worker cannot lose an invalidation against the draw path.
- ShaderObject: compile artifacts (TShader, preprocessed source, side-channel
  maps, status/log, consume-once flag) behind Compiled(); the P0b layer-1
  memo trio deliberately stays outside as the future non-joining
  COMPLETION_STATUS_KHR fast path.
- CompileEnv (new): a GL-thread snapshot of everything the compile pipeline
  used to read live from the backend mid-parse - compute limits (the
  GetIntegeri_v reach-back is gone from the worker path), advertised
  extensions, device quirks, TBuiltInResource inputs. Captured lazily per
  backend activation; the consume-once re-parse now runs against the same
  env as the original parse.
- The GL-thread prologue / worker-body boundary is marked in Link() where
  the stage sort ends; everything below is a pure function of the snapshot.

Public getter signatures unchanged - MG_Impl and both backends compile
untouched. Unit 476/476, Program suites 117/117, DirectGLES retrace 38/39 on
llvmpipe (the one failure is the known pre-existing non-CI iterationrp case;
the NVIDIA userspace driver was updated out from under the running kernel
module mid-session, so GLX there is down until a reboot).

MobileGL/MG_State/GLState/Core.cpp

@@ -9,6 +9,8 @@
 #include "Core.h"
 #include "MG_State/GLState/RenderbufferState/RenderbufferObject.h"
 #include "MG_State/EGLState/Core.h"
+#include <MG_Backend/BackendObjects.h>
+#include <MG_Util/ShaderTranspiler/CompileEnv.h>
 #include <Config.h>
 
 namespace MobileGL::MG_State {
@@ -24,6 +26,18 @@ namespace MobileGL::MG_State {
     }
 
     namespace GLState {
+        const SharedPtr<const MG_Util::ShaderTranspiler::CompileEnv>& GLContext::GetCompileEnv() {
+            const void* backend = static_cast<const void*>(MG_Backend::pActiveBackendObject.get());
+            if (!m_compileEnv || m_compileEnvBackend != backend) {
+                // First use, or the backend was swapped underneath us. Re-capturing rolls the
+                // fingerprint, so every P0b preprocess memo computed against the old backend's
+                // limits becomes structurally unreachable instead of silently reusable.
+                m_compileEnv = MG_Util::ShaderTranspiler::CaptureCompileEnv();
+                m_compileEnvBackend = backend;
+            }
+            return m_compileEnv;
+        }
+
         // Error
         void GLContext::RecordError(ErrorCode code, UniquePtr<ErrorInfo> info) {
             m_errorState.RecordError(code, Move(info));