fix: a late frame is now dropped before it reaches the timeline, leaving its reserved hole intact

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/acquisition.cpp

@@ -462,6 +462,16 @@ void Acquisition::on_data(const udps_frame_t* frame) {
     have_counter_ = true;
   }
 
+  // A late frame's samples belong in the hole already reserved for them, not at the end of
+  // the timeline. Appending them would walk the time ring's clock backwards, which trips the
+  // non-monotonic guard and can throw the whole stream onto the arrival clock -- a far worse
+  // outcome than the dropped packet we used to have. Until the backfill lands (RingBuffer::amend
+  // and MinMaxPyramid::amend), late frames are dropped here, leaving the reserved hole holding
+  // the NaN it was reserved with -- a value every reader already treats as absent. This is
+  // exactly the behaviour the client had before it started delivering late packets, so nothing is
+  // lost relative to that. This return is the single place the backfill will later hook into.
+  if (frame->late) return;
+
   const Partition& part = model_.partition();
   const std::vector<ChannelInfo>& channels = model_.channels();
   if (part.time_capacity == 0 || channels.empty()) return;