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.

include/plot/render/shaders.hpp

@@ -17,6 +17,11 @@ namespace plot::render {
 /// `plot::Rect` is top-left origin while GL is bottom-left, so the y axis is
 /// flipped exactly once, here.
 ///
+/// A degenerate segment (`a_p0 == a_p1`) keeps a finite normal and is stretched
+/// into a square mark one stroke width and fringe across, rather than
+/// collapsing to zero area and rasterising nothing — the dense path stages
+/// `min == max` buckets as exactly such segments.
+///
 /// The fragment shader discards outside the fringe and ramps alpha over the last
 /// pixel of the half width with `smoothstep`, which is what keeps a 1px trace
 /// from aliasing into dashes.