wip(plot): region layout, hot handles and draggable handles (build restored)

include/plot/render/style.hpp

@@ -68,6 +68,13 @@ struct Style {
   bool grid_minor_visible = true;  ///< stage the minor grid at all.
   f32 line_width_px = 1.5f;        ///< logical width of the strokes framing the data
                                    ///< (axis lines, cursor markers).
+  /// Logical width of a *draggable handle* drawn highlighted: the tab a hovered or
+  /// grabbed cursor marker is widened to, and the tab a region's hot bound is drawn
+  /// as -- the latter sized by `Plot::layout()` with this same metric, so the two
+  /// kinds of handle are one width. Wider than `line_width_px` on purpose: the cue
+  /// for "this marker will follow your drag" has to be visible on a marker whose
+  /// own colour may already be the accent.
+  f32 handle_width_px = 5.0f;
   f32 tick_min_px = 60.0f;         ///< logical minimum spacing between major ticks, so
                                    ///< a 2x display gets the same tick *count*.