[Fix] (Pipe, Purity): end a declared verb honestly, and gate the header MG_State now includes

- MGPipeLeaveVerb() bumps the serial and puts the current verb back to none, so a test that
  drives a backend helper directly stops declaring where it says it stops and a later
  unguarded read aborts as "<Field>@<none>" instead of naming an unrelated verb
- check_include_closure.py gains a fourth probe: F2 put MGP_NOTE_MUTATION into frontend
  mutators, so MG_State includes MG_Pipe/PipeMutation.h and that header must never reach
  back into MG_State, MG_Impl or a backend

scripts/check_include_closure.py

@@ -106,6 +106,25 @@ PROBES = [
         "Why": "P7 ships the reflection artifacts over the wire; the archive header must not "
                "depend on the compiler front end that produced them.",
     },
+    {
+        "Name": "mutation-header",
+        "Header": "MobileGL/MG_Pipe/PipeMutation.h",
+        "Tu": "#include <MG_Pipe/PipeMutation.h>\n",
+        "Forbidden": [
+            "MobileGL/MG_Impl/",
+            "MobileGL/MG_State/GLState/Core.h",
+            "MobileGL/MG_Remote/",
+        ],
+        "Allow": [],
+        "TextLimits": {},
+        "Why": "P1 finding F2 put MGP_NOTE_MUTATION in frontend mutators, so MG_State now "
+               "includes this header (TextureState.h, SamplerObject.h). It may only DECLARE "
+               "the notice: reaching MG_Impl would pull the fill implementation into the "
+               "state machine that calls it, and reaching GLState/Core.h would close the "
+               "cycle back onto the context. The MGPipeTypes -> BackendObject -> TextureEnum "
+               "reach it inherits from MGPipe.h is D1's known exception, which is why gate A "
+               "asserts MGPipeValueTypes.h rather than MGPipeTypes.h.",
+    },
     {
         "Name": "wire-header",
         "Header": "MobileGL/MG_Remote/Transport/ITransport.h",