feat: relative mode keeps the stream's absolute clock for the export

src/event_series.hpp

@@ -32,6 +32,10 @@ struct EventSnapshot {
   /// (see `BufferStore::time()`), which also makes the cut in `publish_event()`/
   /// `publish_pause_view()` a straight copy.
   std::vector<f64> stamps;
+  /// Absolute stream seconds on the same indices as `stamps`, or empty when the store had no
+  /// absolute ring. **Not** rebased by `x_offset` or by the trigger edge: a snapshot outlives
+  /// the epoch that produced it, so it stores the absolute value outright.
+  std::vector<f64> abs_stamps;
   std::vector<std::vector<f32>> values;  ///< one array per channel, same length
   /// The stream time of this snapshot's x = 0, so `x_offset + stamps[i]` is the clock the
   /// data arrived on: the edge for a trigger event, the newest sample at the moment of the