[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

MobileGL/MG_Impl/Pipe/PipeFill.cpp

@@ -552,6 +552,16 @@ namespace MobileGL::MG_Pipe {
         ctx->RecordError(code, Move(info));
     }
 
+    void MGPipeLeaveVerb() {
+        PipeInputs& inputs = gPipeInputs;
+#if MOBILEGL_PIPE_POISON
+        // Same bump the next fill would make, without a verb to fill from: no field is
+        // stamped, so every stamp this verb made falls behind the serial.
+        ++MGPipeFillAccess::Filled(inputs).CurrentVerbSerial;
+#endif
+        MGPipeFillAccess::SetVerb(inputs, MGPipeVerb::kVerbCount);
+    }
+
     // ---- the filler ----
     void MGPipeFillForVerb(MGPipeVerb verb) {
         PipeInputs& inputs = gPipeInputs;

MobileGL/MG_Impl/Pipe/PipeFill.h

@@ -24,6 +24,15 @@ namespace MobileGL::MG_Pipe {
     // runs the entry compare against a second snapshot (P1 brief D8).
     void MGPipeFillForVerb(MGPipeVerb verb);
 
+    // Ends the verb in flight without starting another: bumps the serial, so every field the
+    // verb stamped goes stale, and puts the current verb back to "none", so a read made after
+    // it aborts as Fatal{UnmigratedPipeInput, "<Field>@<none>"} - which is what such a read
+    // is - instead of naming whichever verb happened to be filled last. Nothing in the GL
+    // entry points calls this: a real verb is always followed by the next verb's fill. It
+    // exists for a caller that drives a backend helper directly and wants its declaration to
+    // stop where it says it stops (MG_Test/ScopedPipeVerb.h).
+    void MGPipeLeaveVerb();
+
     // PipeFill.cpp. Negative control B (P1 brief D6): the filler withholds the STAMP - never
     // the value - of `field` at `verb`, so that verb's read of it is
     // Fatal{UnmigratedPipeInput, "Field@Verb"} while every other verb is unaffected. The