fix(plot,glfw): layer the crosshair under the handles, sync it across a figure, and harden the lookup

include/plot/plot/crosshair.hpp

@@ -87,7 +87,7 @@ struct CrosshairSpan {
 /// The sample span `x` falls in, on `series`; see `CrosshairSpan` for the rule, for the
 /// interpolation convention it is half of, and for the cases that yield `valid == false`.
 ///
-/// O(1) and allocation-free: a subtraction, a division and a floor. The series' own sample
+/// O(1) and allocation-free: a subtraction, a division and a truncation. The series' own sample
 /// fetch is the caller's -- `Plot::crosshair_value()` reads the span through
 /// `Series::gather()` into a scratch buffer it owns, so no lookup on the pointer path
 /// allocates.