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

tests/test_primitives.cpp

@@ -43,6 +43,30 @@ PLOT_TEST(line_batch_stages_segment_endpoints) {
   PLOT_CHECK_NEAR(b.data()[7], 1.0, 0.0);
 }
 
+// `add_polyline_xy` takes the layout `Series::gather` appends to, so this pins
+// that a gathered window becomes the segments it says it does -- an off-by-one in
+// the stride here would draw a diagonal where the data has a corner.
+PLOT_TEST(line_batch_stages_interleaved_points) {
+  plot::render::LineBatch b;
+  b.begin();
+  const plot::f32 xy[8] = {0.0f, 1.0f, 2.0f, 3.0f, 4.0f, 5.0f, 6.0f, 7.0f};
+  b.add_polyline_xy(xy, 4);  // 3 segments
+  PLOT_CHECK(b.segment_count() == 3);
+  PLOT_CHECK(b.vertex_count() == 6);
+  PLOT_CHECK(b.data().size() == 12);
+  PLOT_CHECK_NEAR(b.data()[0], 0.0, 0.0);
+  PLOT_CHECK_NEAR(b.data()[1], 1.0, 0.0);
+  PLOT_CHECK_NEAR(b.data()[2], 2.0, 0.0);
+  PLOT_CHECK_NEAR(b.data()[3], 3.0, 0.0);
+  PLOT_CHECK_NEAR(b.data()[10], 6.0, 0.0);
+  PLOT_CHECK_NEAR(b.data()[11], 7.0, 0.0);
+  // Fewer than two points, or none: nothing to draw, and nothing to crash on.
+  b.clear();
+  b.add_polyline_xy(xy, 1);
+  b.add_polyline_xy(nullptr, 4);
+  PLOT_CHECK(b.vertex_count() == 0);
+}
+
 PLOT_TEST(line_batch_ignores_degenerate_polylines) {
   plot::render::LineBatch b;
   b.begin();