Bound the automation hold to the window the model has not caught up on, and make that authority model stated, enforced and tested

src/core/instrument/engine/live_params.h

@@ -71,6 +71,16 @@ struct LiveValues {
 static_assert(std::is_trivially_copyable_v<LiveValues>,
               "the live block is copied under a seqlock — it must stay a plain value");
 
+// FIELD-wise equality, and it must never be "simplified" into a memcmp. LiveValues carries
+// padding, and nothing gives that padding a determinate value across a copy: NRVO is optional
+// and the implicit copy/move is specified member-wise, so two blocks folded from the same
+// parameter set are NOT reliably byte-equal. A byte compare therefore reports differences that
+// do not exist — which is exactly what it did before this existed. Listed member by member, so a
+// member added to the block above must be added here as well; this sits directly beneath the
+// struct for that reason.
+bool operator==(const LiveValues& a, const LiveValues& b);
+inline bool operator!=(const LiveValues& a, const LiveValues& b) { return !(a == b); }
+
 // The ONE derivation of the live block from the parameter set. Every publisher goes through
 // here so there is a single site to keep in step with PlayParams. `keyTrack` is passed in
 // because it belongs to the capture/instrument scalar beside the play bundle, not to