fix: a loaded recording is filtered into its buffer, not truncated

The loader pushed one slot per file row while the plan decimated by
eight, so a 10 M-row file overran the ring and only its last 4.19 s
survived -- a 20 s recording that read as a 4 s one. Each group of
`time_stride` rows is now averaged into one slot at the group's mean
stamp, as the packet path already does, and the load line reports
rows and decimation so a filtered file cannot be mistaken for a
truncated one.

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

src/main.cpp

@@ -76,11 +76,15 @@ int main(int argc, char** argv) {
     // The whole file framed from the start: the window is the recording's own span, and nothing
     // afterwards restates it -- a zoom is the user's.
     if (report.span_s > 0.0) rows.set_time_window(report.span_s);
+    // `rows`/`decim` say whether what is on screen is the file sample for sample or a filtered
+    // version of it: `samples` alone below `rows` is how a recording that did not fit its buffer at
+    // its own rate used to read as a truncated one.
     std::fprintf(stderr,
-                 "[udpscope] waveform loaded: %s signals=%zu samples=%zu span=%.6fs rate=%.6g "
-                 "time=[%.6f..%.6f]\n",
-                 cfg.open_waveform.c_str(), report.signals, report.samples, report.span_s,
-                 report.rate_hz, report.first_time, report.last_time);
+                 "[udpscope] waveform loaded: %s signals=%zu samples=%zu rows=%zu decim=%llu "
+                 "span=%.6fs rate=%.6g time=[%.6f..%.6f]\n",
+                 cfg.open_waveform.c_str(), report.signals, report.samples, report.rows,
+                 static_cast<unsigned long long>(report.decimation), report.span_s, report.rate_hz,
+                 report.first_time, report.last_time);
     std::fflush(stderr);
   }