feat: relative mode stamps a sample from its position and the declared rate

src/acquisition.hpp

@@ -90,6 +90,17 @@ class Acquisition {
   /// signal carrying volt-like values, others from the arrival clock -- which is a
   /// time base no reader can make sense of (see `build_stamps`).
   int time_source_ = 0;
+  /// Relative-time mode's position counter: samples seen since the stream began, which is
+  /// what the counter names. Advanced by `batch_` for every packet the counter reports
+  /// missing (before the frame's own samples) and then by the frame's own `n`, so it is
+  /// exactly `counter * batch + sample_index` for a constant batch and degrades gracefully
+  /// when a frame's length disagrees.
+  std::uint64_t slot_base_ = 0;
+  /// Samples per DATA packet, fixed by the first frame. Only ever needed to size what is
+  /// *absent*: a delivered frame carries its own length. 0 until the first frame.
+  std::uint32_t batch_ = 0;
+  /// Latches the one warning about a frame whose length disagrees with the stream's first.
+  bool warned_batch_ = false;
   std::vector<double> stamps_;
   /// The decimation filter's state: a running sum per channel (and one for the shared
   /// clock) over the group of base samples that becomes one stored sample. A boxcar