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

include/plot/render/renderer.hpp

@@ -106,8 +106,17 @@ class Renderer {
   /// physical size on a 1x panel and a 2x one.
   ///
   /// `Plot::layout()` is computed exactly once, here, and handed to every helper:
-  /// it runs the auto scale, the strip fixed-point loop and the grid build, so
-  /// re-deriving it per helper would rebuild the ticks and the grid per helper.
+  /// it runs the auto scale, the strip fixed-point loop, the grid build and the
+  /// region geometry, so re-deriving it per helper would rebuild all of that per
+  /// helper.
+  ///
+  /// The handles a pointer is on are drawn highlighted -- a cursor marker is widened
+  /// into a tab, a region bound is drawn in the style's cursor colour at the width
+  /// `PlotLayout` sized its handle at -- and what is hot comes from
+  /// `Plot::hot_handles()`, which the input layer publishes. The renderer therefore
+  /// never sees a pointer, a button or a gesture: it draws what the plot says is
+  /// being manipulated, and only when a host drives the plot with a controller is
+  /// anything ever hot.
   ///
   /// A legend the plot has is part of the frame: the panel and its swatches are
   /// line geometry staged with everything else, the names are glyphs on the text
@@ -115,6 +124,11 @@ class Renderer {
   /// line batch, so the legend cannot be drawn under a trace, and it leaves no GL
   /// state of its own -- the frame's blend and its neighbours' scissor already say
   /// everything a fill and a hairline border need (see end_frame's residue list).
+  ///
+  /// Regions are part of that same evidence: the fill is one instance of the line
+  /// pipeline (`fill_pixels()`, the primitive the legend panel uses), the bounds are
+  /// vertical strokes and the label is a staged glyph run, so a frame with regions
+  /// leaves behind exactly what a frame without them does.
   void draw(Plot& plot);
 
   /// Ends the frame, draining `glGetError()` in a loop so a stale error cannot be
@@ -316,6 +330,16 @@ class Renderer {
   /// pins.
   void draw_traces(const Plot& plot, const PlotLayout& layout);
   void draw_cursors(const Plot& plot, const PlotLayout& layout);
+  /// The regions: the fill, one bound per edge, the optional label. Every rect comes
+  /// from `PlotLayout::regions` -- the same geometry the controller hit-tests -- so
+  /// a region is drawn where it can be grabbed, and neither side derives the other's
+  /// rects.
+  void draw_regions(const Plot& plot, const PlotLayout& layout);
+  /// One region bound: a vertical stroke of `width` down the tab's own column, in
+  /// `color`. The column is `handle_column()` of the tab -- the bound the layout
+  /// placed -- and a bound outside the axes' columns is skipped, exactly as a cursor
+  /// marker out of the visible window is.
+  void draw_region_edge(const Rect& tab, const Rect& axes, Color color, f32 width);
   void draw_measures(const Plot& plot, const PlotLayout& layout);
   void draw_annotations(const Plot& plot);
   /// The legend: the panel and its swatches as line geometry, the names on the text