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>

docs/interaction.md

@@ -113,9 +113,10 @@ across the figure, and `Figure::set_crosshair(x, hovered_row, y_in_row, active)`
 horizontal line **only in the row the pointer is in** — a figure's rows share x and not y, so
 a figure-wide horizontal line would be a value that means something different in each row.
 
-Readout values are **interpolated**, not interval statistics (see
-[data-and-performance.md](data-and-performance.md)); a row whose trace has no sample at the
-crosshair's x is listed with the em dash `"—"` and no marker, rather than a fabricated value.
+Readout values are the **nearest sample's**, neither interpolated between two samples nor an
+interval statistic (see [data-and-performance.md](data-and-performance.md)); a row whose trace
+has no sample at the crosshair's x is listed with the em dash `"—"` and no marker, rather than a
+fabricated value.
 
 ## The gesture table (exactly as implemented)