fix: the t0 caption is chrome anchored to the axis, not an annotation in the data

The caption that names the relative x labels' anchor was an `Annotation`, whose anchor
is a data coordinate -- so it crawled across the plot under a pan and drifted under a
zoom, which is how it was reported. It is now `Plot::set_x_note()`: the bottom row's
caption, drawn by the renderer in the axes' bottom-left corner in pixels, on the
legend's own backplate.

The text stays the host's, because only the host knows that label + caption has to be
the exact absolute time (`%.6g`, where `format_time()` would round to three figures).

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

src/ui.cpp

@@ -7264,10 +7264,7 @@ gboolean timelabels_probe_step(gpointer user) {
         ticks += t.label;
       }
     }
-    if (last.time_origin_note < last.plot->annotation_count()) {
-      const plot::Annotation& an = last.plot->annotation(last.time_origin_note);
-      if (an.visible) note = an.text;
-    }
+    note = last.plot->x_note();
   }
   double first_value = 0.0;
   plot::Range shown{};
@@ -8317,14 +8314,12 @@ int run_ui(int argc, char** argv, const Config& cfg, ScopeModel& model, Acquisit
     }
     {
       // The x label form: relative to the window's own start (with the caption that names
-      // that start) while the span is sub-second, absolute otherwise. `x_origin_set` is read
-      // back out of the plot, so this is what the axis is actually formatting with.
+      // that start) when the labels could not carry the tick step absolutely, absolute
+      // otherwise. `x_origin_set` is read back out of the plot, so this is what the axis is
+      // actually formatting with, and `note` is the plot's own caption string.
       const RowSet::Row& rlt = rows.row(rows.row_count() - 1);
       std::string note;
-      if (rlt.plot != nullptr && rlt.time_origin_note < rlt.plot->annotation_count()) {
-        const plot::Annotation& a = rlt.plot->annotation(rlt.time_origin_note);
-        if (a.visible) note = a.text;
-      }
+      if (rlt.plot != nullptr) note = rlt.plot->x_note();
       // The labels themselves, from the axis the reader looks at: the assertion is what the
       // ticks *say*, not only which mode is on.
       std::string ticks;