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

src/plot/plot.cpp

@@ -284,6 +284,16 @@ const Measurement& Plot::measurement(u32 id) const {
 
 void Plot::invalidate_measurements() noexcept { mark_measures_dirty(); }
 
+MeasureSpan Plot::measure_span(u32 id) const noexcept {
+  // An id that names nothing, or a measure built from cursor ids that were never
+  // registered, is not a span between two markers: `valid` says so rather than
+  // letting a caller shade a band between cursors 0 and 1 by accident.
+  if (id >= measures_.size()) return MeasureSpan{};
+  const MeasureSlot& slot = measures_[id];
+  if (slot.cursor_a >= cursors_.size() || slot.cursor_b >= cursors_.size()) return MeasureSpan{};
+  return MeasureSpan{slot.cursor_a, slot.cursor_b, true};
+}
+
 const PlotLayout& Plot::layout() {
   // Axes still on auto scale refit here, once per layout, so a growing stream
   // stays framed. A caller gesture clears auto_scale (set_x_range/set_y_range),