fix: a cursor readout quotes the nearest stored sample, not a value between two

The crosshair and the cursors pane's mix values are one library call, and it
interpolated linearly between the two samples bracketing the pointer. That is
wrong twice over here. Ingest decimation is a boxcar average over `time_stride`
samples, so a number computed between two stored samples is a second, undeclared
estimate laid over the first that nothing on screen or in an export holds. And
past the sample spacing the polyline is a line the renderer drew, so quoting a
point on it reports the drawing rather than the data.

The fix is upstream and unconditional (plotter c028f48): `crosshair_span()`'s `t`
selects the nearer end instead of blending, with a tie reading the earlier
sample. `CrosshairRow::marker` is placed from the same y, so the dot follows the
quoted sample.

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

src/channel.hpp

@@ -73,8 +73,8 @@ class ChannelBuffer final : public plot::data::Series {
   /// datagram leaves a gap, so index times must come from the time ring.
   f64 x_at(u64 i) const override;
   /// Bracketing samples by binary search over the time ring, which is monotonic,
-  /// so a crosshair value is interpolated between the two samples actually either
-  /// side of the pointer -- a grid walk would land on the wrong pair after a gap.
+  /// so a crosshair reads whichever of the two samples actually either side of the
+  /// pointer is nearer -- a grid walk would land on the wrong pair after a gap.
   bool neighbours(f64 x, u64& i0, u64& i1) const override;
   u64 snap_index(f64 x) const override;
   plot::Range x_bounds() const override;

src/help/09-cursors.txt

@@ -19,6 +19,9 @@ owns a <b>Reset</b> button. Drag a marker to move it. The mode dropdown offers f
 • <tt>mix</tt> is the reading mode: one line per <i>bound</i> trace, with the trace's own values
   at each marker and their difference. A slot with a name but no channel behind it is not a
   trace and gets no line.
+• A value is the <i>nearest stored sample's</i>, never a number between two samples. Zoom in past
+  the sample spacing and the line between two samples is something the plot drew, not data —
+  so a reading off it would be the drawing. The hover crosshair reads the same way.
 • A marker past the newest sample reads <tt>--</tt>. With only one marker over data the row
   reads <tt>partial</tt>: a difference between a number and nothing is not a number.
 • In <tt>mix</tt> each marker also places a crossing annotation per trace, which is what lets