fix(render): give degenerate segments area, pin font bit order, clarify vertex_count

include/plot/render/primitives.hpp

@@ -34,7 +34,11 @@ class LineBatch {
 
   /// Segments staged so far, one instance each.
   u32 segment_count() const noexcept { return static_cast<u32>(v_.size() / 4u); }
-  /// Vertices the pipeline will rasterise: two per staged segment.
+  /// Staging vertices, two per staged segment. The pipeline draws one instanced
+  /// quad per segment (its four `GL_TRIANGLE_STRIP` corners come from
+  /// `gl_VertexID`), so the instance count a draw passes is `vertex_count() / 2`
+  /// — handing `vertex_count()` to `glDrawArraysInstanced` would rasterise
+  /// twice the geometry the batch staged.
   u32 vertex_count() const noexcept { return segment_count() * 2u; }
 
   /// Interleaved `x0,y0,x1,y1`, four floats per staged segment.