[Docs] (Pipe): record why the forwarders carry no poison check, the InHook re-entry guard, and the Index slot no FillPoints.def row can fill

- The seven F-class forwarders are the declared exception to D4's "every accessor body": a forward is a live call, not a stored value, and InvalidateCompileEnv is reached from backend initialisation before any verb has filled, where a check would be Fatal{...@<none>} on every start. Their sticky stamp is consulted by no accessor; the tests pin it through MGPipeInputFieldIsFresh directly. Written down so P2's tracker does not "fix" the missing check.
- MGPipeVerifyReadHook: InHook is what keeps a GetProgramForDraw-triggered backend re-entry from recursing into a second hook, and the whole-field re-read is a per-read cost to keep in mind when reading the verify lane's wall time.
- GetBufferBindingSlot(Index) is polymorphic on GLContext (the bound VAO's element-buffer slot) and null in the block by design: a push Fatal there is not a missing row. No backend reads it today.

MobileGL/MG_Backend/MGPipe/PipeInputs.h

@@ -560,6 +560,12 @@ namespace MobileGL::MG_Pipe {
         // i.e. it is a lookup or a reverse-channel write, not a state read; there is no value
         // the filler could copy and no verb whose fill could make it stale. Phase C replaces
         // them with handle tables and callbacks.
+        // They carry no MGP_INPUT_CHECK / MGP_INPUT_VERIFY_READ (the declared exception to
+        // P1 brief D4's "every accessor body"): a forward is a live call, not a stored value,
+        // and InvalidateCompileEnv is reached from backend initialisation before any verb has
+        // filled, where a check would be Fatal{...@<none>} on every start. Their sticky stamp
+        // is therefore consulted by no accessor; the tests pin it through
+        // MGPipeInputFieldIsFresh directly.
         SizeT GetBufferBindingPointCount(BufferTarget target) const;
         const SharedPtr<ProgramObject>& GetProgramObject(Uint index);
         const SharedPtr<ITextureObject>& GetTextureObject(Uint index);