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

src/acquisition.hpp

@@ -102,6 +102,7 @@ class Acquisition {
   /// Latches the one warning about a frame whose length disagrees with the stream's first.
   bool warned_batch_ = false;
   std::vector<double> stamps_;
+  std::vector<double> abs_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
   /// average, which is what turns decimation into a *resampling* rather than a stride:
@@ -114,6 +115,7 @@ class Acquisition {
   std::vector<f32> math_sample_;
   std::vector<std::uint64_t> value_acc_n_;
   double time_acc_ = 0.0;
+  double abs_time_acc_ = 0.0;
   std::uint64_t time_acc_n_ = 0;
   /// The partition the accumulators were sized for, so a re-plan resets them rather than
   /// averaging across the boundary.