[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.h

@@ -21,6 +21,10 @@
 #include "VertexArrayState/VertexArrayState.h"
 #include "RenderbufferState/RenderbufferState.h"
 
+namespace MobileGL::MG_Util::ShaderTranspiler {
+    struct CompileEnv;
+}
+
 namespace MobileGL {
     namespace MG_State {
         void Init();
@@ -380,6 +384,15 @@ namespace MobileGL {
                 Bool ValidateRenderbufferName(Uint index) const;
                 Bool ValidateRenderbufferObject(Uint index) const;
 
+                // P1: the shader compile/link pipeline's snapshot of everything it reads from
+                // outside its own (stage, source) inputs. Captured lazily here because it
+                // cannot be captured in MG_State::Init() - that runs BEFORE MG_Backend::Init(),
+                // so there is no backend to query yet. Re-captured whenever the active backend
+                // object changes, which also rolls the fingerprint and therefore invalidates
+                // every P0b preprocess memo keyed against the old one.
+                // GL thread only.
+                const SharedPtr<const MG_Util::ShaderTranspiler::CompileEnv>& GetCompileEnv();
+
             private:
                 // State Components
                 ErrorState m_errorState;
@@ -437,6 +450,11 @@ namespace MobileGL {
                 FramebufferState m_framebufferState;
                 SamplerState m_samplerState;
                 RenderbufferState m_renderbufferState;
+
+                mutable SharedPtr<const MG_Util::ShaderTranspiler::CompileEnv> m_compileEnv;
+                // Identity of the backend object m_compileEnv was captured against; a plain
+                // pointer compare, never dereferenced.
+                const void* m_compileEnvBackend = nullptr;
             };
         } // namespace GLState