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>

include/plot/data/series.hpp

@@ -97,7 +97,7 @@ class Series {
   /// lands exactly on a sample). False when x lies outside the series' x
   /// bounds; `i0`/`i1` are then untouched. The crosshair and measurement
   /// paths read coordinates only through this and `x_at()`, so a series with
-  /// its own x array interpolates exactly.
+  /// its own x array brackets exactly.
   virtual bool neighbours(f64 x, u64& i0, u64& i1) const;
 
   /// Index of the sample nearest data-space `x`, clamped to `[0, size()-1]`;