fix(plot): a crosshair readout quotes a sample, never a value between two

The readout's y was a linear interpolation between the two samples adjacent to
the pointer, which answers "what is the signal at this instant" -- a number the
series does not contain. On a source whose samples are already averaged or
decimated at ingest that is a value nothing measured, and at a zoom past the
sample spacing it is visibly not the datum under the pointer.

`crosshair_span()`'s `t` becomes a selector rather than a blend factor: the
nearer end wins, a tie reading the earlier sample (the bias the span already
takes for a degenerate pair). `t` is measured on the samples' own x through
`neighbours()`/`x_at()`, so "nearer" is exact on a non-uniform clock too, and
the lookup still reads level 0, so the value is a stored sample and never an
LOD bucket's aggregate. `crosshair_interpolate()` had no other caller and is
gone.

`CrosshairRow::marker` is placed from the same y, so the dot follows the
reported sample: past the sample spacing it sits beside the polyline rather
than on it, deliberately -- one y feeding both keeps the mark and the number
from ever disagreeing.

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

tests/test_xy_series.cpp

@@ -72,9 +72,9 @@ PLOT_TEST(xy_series_reports_its_own_grid) {
 }
 
 PLOT_TEST(xy_series_neighbour_lookups_crosshair_style) {
-  // The crosshair asks neighbours() at an arbitrary x and interpolates between
-  // the two samples it gets back. On a non-uniform grid that needs real
-  // searches: a uniform-grid formula would land megabytes off here.
+  // The crosshair asks neighbours() at an arbitrary x and reports whichever of
+  // the two samples it gets back is nearer. On a non-uniform grid that needs
+  // real searches: a uniform-grid formula would land megabytes off here.
   auto x = spread_x(1000);
   const std::vector<f64> xs = x;
   plot::data::XYSeries s(std::move(x), saw_y(1000));