refactor(render): TextBackend owns its atlas, and staged glyphs carry a rotation basis

include/plot/render/vertex_array.hpp

@@ -55,13 +55,16 @@ class LineVertexArray {
 };
 
 /// Vertex array plus instance buffer of the text pipeline: `a_pos` at location 0
-/// and the glyph's atlas texel origin `a_uv` at location 1, two floats each,
-/// advancing once per instance.
+/// (the cell's top-left corner in pixels), the glyph's atlas rect `a_uv` at
+/// location 1 and its rotation basis `a_axis` at location 2, two floats each,
+/// every one advancing once per instance. The layout is the one `TextInstance`
+/// declares and shaders.hpp consumes; a caller never writes a location, stride or
+/// offset, it stages a `TextBatch` and uploads its `data()`.
 class TextVertexArray {
  public:
   bool create(Status* st = nullptr, UploadMode mode = UploadMode::Orphan) noexcept;
 
-  /// Uploads `glyphs` instances from interleaved `x,y,u,v` floats.
+  /// Uploads `glyphs` instances from interleaved `x,y,u,v,ax,ay` floats.
   void upload(const f32* interleaved, u32 glyphs) noexcept;
 
   void bind() const noexcept { vao_.bind(); }