[Feat] (Pipe): fill PipeInputs per verb class from GLContext and stamp per-verb generations - a read of a field the verb did not fill is Fatal{UnmigratedPipeInput}

- MGPipeFillForVerb now walks kMGPipeClassFieldMask[kMGPipeVerbClass[verb]] and copies every field in it by calling the GLContext accessor of the same name (MGPipeFillAccess::CopyField, one switch over the 56 stored fields; the seven forwarded fields copy nothing), stamping each with the new serial; the sticky seven get FilledGen = 1 on the first live fill through the same mask walk, since every class mask carries them.
- MOBILEGL_PIPE_POISON_OMIT (<Verb>:<FieldName>) is parsed once on the first fill and MGPipeSetPoisonOmission is the programmatic form for the unit tests; the omitted pair keeps its value copy and loses only its stamp, so the omission is indistinguishable from a forgotten FillPoints.def row. An unknown name is Fatal{PipeVerifyBadKnob}; a non-poison push build acknowledges the knob with one warning because no stamp exists to omit.
- PipeInputs names one friend, struct MGPipeFillAccess, instead of two friend functions, and VisitStorage gains a const overload for the comparator that follows.

MobileGL/MG_Backend/MGPipe/PipeInputs.h

@@ -567,6 +567,18 @@ namespace MobileGL::MG_Pipe {
     case MGPipeInputField::Field:                                                                                      \
         return fn(a.Member, b.Member);
                 MGP_INPUT_STORAGE_LIST(MGP_INPUT_VISIT)
+#undef MGP_INPUT_VISIT
+            default:
+                return false;
+            }
+        }
+        template <class Fn>
+        static Bool VisitStorage(MGPipeInputField field, const PipeInputs& a, const PipeInputs& b, Fn&& fn) {
+            switch (field) {
+#define MGP_INPUT_VISIT(Field, Member)                                                                                 \
+    case MGPipeInputField::Field:                                                                                      \
+        return fn(a.Member, b.Member);
+                MGP_INPUT_STORAGE_LIST(MGP_INPUT_VISIT)
 #undef MGP_INPUT_VISIT
             default:
                 return false;
@@ -574,8 +586,9 @@ namespace MobileGL::MG_Pipe {
         }
 
     private:
-        friend void MGPipeFillForVerb(MGPipeVerb verb);
-        friend void SnapshotFromGLContext(PipeInputs& snapshot, const MGPipeFieldMask& mask);
+        // The one door into the storage from the client side (MG_Impl/Pipe/PipeFill.cpp):
+        // the filler's per-field copies and stamps, and the verify snapshot.
+        friend struct MGPipeFillAccess;
 
         // ---- identity ----
         const void* m_contextIdentity = nullptr;