feat(render): Renderer with LOD selection, 3.3 and 4.3+ upload paths, headless smoke test

include/plot/plot/plot.hpp

@@ -58,6 +58,10 @@ class Plot {
   /// creates one and never touches GL.
   void set_text_backend(const render::TextBackend* text) noexcept { text_ = text; }
 
+  /// The backend layout() measures labels with, or null when none is set. The
+  /// renderer reads it to install its own font only when nobody else has.
+  const render::TextBackend* text_backend() const noexcept { return text_; }
+
   // --- construction -----------------------------------------------------
 
   u32 add_x_axis(const AxisDesc& desc);
@@ -135,6 +139,11 @@ class Plot {
   /// because recomputing fills a cache: nothing a caller can observe changes.
   const Measurement& measurement(u32 id) const;
 
+  /// The cursor pair `id` was created over. The renderer needs it to place the
+  /// shaded band the measurement readout belongs to; the statistics themselves
+  /// are in measurement().
+  MeasureSpan measure_span(u32 id) const noexcept;
+
   /// Drops every cached measurement. Needed after a series is mutated in place
   /// (data::ArraySeries::commit() with the same sample count), which a pointer
   /// and size check cannot detect.