feat(plot): the readout marker snaps to the sample it quotes, and the value mode is a choice

The mark was placed at the *pointer's* x with the quoted sample's y, so with the
nearest-sample readout it sat off the datum horizontally -- and further off the deeper
the zoom. `CrosshairRow` now carries the data `x` the value belongs to and the marker is
placed from it, so in `NearestSample` the mark lands on the sample the panel is quoting.
`Style::crosshair_marker_px` (9) and `crosshair_marker_width` (1.5) replace the
renderer's hard-coded 3 px arm and 1 px stroke: a snapped mark lands among a trace's own
sample dots (`marker_px`, 3) and at that size would read as one more sample rather than
as the answer to the question the hover asks.

`CrosshairValue` makes the previous behaviour reachable rather than deleted. The two
modes answer different questions -- `NearestSample` "which datum is under the pointer",
`Interpolated` "what is the signal at this instant" -- and only the caller knows which it
is asking, so the plot offers both with the sample as the default.
`crosshair_interpolate()` is restored for it.

`Plot::crosshair_point()` is the lookup with both coordinates, public because a host that
draws instruments of its own needs the same snap and a second rule for it would drift
from this one; `crosshair_value()` delegates to it.

src/render/renderer.cpp

@@ -1522,19 +1522,28 @@ void Renderer::draw_crosshair_lines(const Plot& plot, const PlotLayout& layout)
     flush_lines(data, style_.crosshair, stroke, crosshair.dash);
   }
 
-  // A small cross at each trace's intersection, in the trace's own colour -- opaque, like a
-  // legend's swatch, so a trace drawn at 15% still has a marker that can be seen. The arm
-  // length is a logical size like every other marker's, and the position is the layout's: a
-  // marker for a sample far outside the visible y range is staged where the trace is and the
-  // axes clip keeps it inside, rather than being skipped by a range test of this layer's own.
-  const f32 arm = style_.device(3.0f);
-  for (const CrosshairRow& row : crosshair.rows) {
-    if (!row.valid) continue;
-    Color color = plot.trace(row.trace).color;
-    color.a = 1.0f;
-    lines_.add_segment({row.marker.x - arm, row.marker.y}, {row.marker.x + arm, row.marker.y});
-    lines_.add_segment({row.marker.x, row.marker.y - arm}, {row.marker.x, row.marker.y + arm});
-    flush_lines(pixel_mapping(), color, style_.device(1.0f));
+  // A cross at each trace's readout point, in the trace's own colour -- opaque, like a legend's
+  // swatch, so a trace drawn at 15% still has a mark that can be seen. The position is the
+  // layout's, and in the default `CrosshairValue::NearestSample` that is the *quoted sample*
+  // rather than the pointer's column: a mark for a sample outside the visible range is staged
+  // where that sample is and the axes clip keeps it inside, rather than being skipped by a range
+  // test of this layer's own.
+  //
+  // `crosshair_marker_px` is the cross' full extent and is larger than `marker_px`, which dots a
+  // trace's own samples at a deep zoom -- a snapped mark lands among those dots and has to read
+  // as the one the panel is quoting. `0` across draws nothing, so a caller can have the panel
+  // and the lines without the marks.
+  const f32 arm = 0.5f * style_.device(style_.crosshair_marker_px);
+  const f32 mark_stroke = style_.device(style_.crosshair_marker_width);
+  if (arm > 0.0f) {
+    for (const CrosshairRow& row : crosshair.rows) {
+      if (!row.valid) continue;
+      Color color = plot.trace(row.trace).color;
+      color.a = 1.0f;
+      lines_.add_segment({row.marker.x - arm, row.marker.y}, {row.marker.x + arm, row.marker.y});
+      lines_.add_segment({row.marker.x, row.marker.y - arm}, {row.marker.x, row.marker.y + arm});
+      flush_lines(pixel_mapping(), color, mark_stroke);
+    }
   }
 }