feat(plot): per-trace legend with measured layout and swatches

include/plot/render/renderer.hpp

@@ -101,6 +101,13 @@ class Renderer {
   /// `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.
+  ///
+  /// 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
+  /// path, and both read the boxes `PlotLayout::legend` measured. It is the last
+  /// 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).
   void draw(Plot& plot);
 
   /// Ends the frame, draining `glGetError()` in a loop so a stale error cannot be
@@ -254,6 +261,14 @@ class Renderer {
   /// is texture-bound, so this runs once per frame, after all line geometry.
   void flush_text() noexcept;
 
+  /// Fills `rect` -- in framebuffer pixels, like `Rect` -- with `color`, in one
+  /// instance: a horizontal stroke whose width is the rect's height *is* the rect,
+  /// because a stroke's quad is its perpendicular width and the fragment shader
+  /// ramps on that perpendicular alone -- the ends are not capped (see
+  /// `fill_pixels()` in the implementation for why a per-column fill, the way the
+  /// measurement band fills, is the wrong primitive here). A rect with no width or
+  /// no height stages nothing.
+  void fill_pixels(const Rect& rect, Color color);
   /// Stages one data-space segment, taking the two axes' log flags into account.
   void stage_segment(f64 x0, f64 y0, f64 x1, f64 y1, bool log_x, bool log_y);
   /// Stages `n` interleaved `x,y` data-space points (a gathered window).
@@ -290,6 +305,11 @@ class Renderer {
   void draw_cursors(const Plot& plot, const PlotLayout& layout);
   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
+  /// path. It reads the boxes `Plot::layout()` measured rather than deriving any of
+  /// them again, so the panel a test asserts and the panel this draws are the same
+  /// numbers.
+  void draw_legend(const Plot& plot, const PlotLayout& layout);
 
   /// The built-in backend the renderer owns. Every use of it goes through the
   /// `TextBackend` interface -- `self`-typed nowhere -- so swapping it for