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

include/plot/render/primitives.hpp

@@ -31,6 +31,10 @@ class LineBatch {
   /// Stages `n - 1` segments joining consecutive points. A null pointer or
   /// fewer than two points stages nothing.
   void add_polyline(const Vec2* pts, u32 n);
+  /// Same, for `n` points already interleaved as `x,y` floats -- the layout
+  /// `data::Series::gather` appends to, so a gathered window is staged without a
+  /// copy or a reinterpret_cast of its storage.
+  void add_polyline_xy(const f32* xy, u32 n);
 
   /// Segments staged so far, one instance each.
   u32 segment_count() const noexcept { return static_cast<u32>(v_.size() / 4u); }