fix(text): carry a glyph's negative bearing and harden the bake's extents

include/plot/render/renderer.hpp

@@ -98,9 +98,12 @@ class Renderer {
   /// which is the same lifetime rule `Plot::set_text_backend()` states.
   void set_text_backend(const render::TextBackend* font) noexcept;
 
-  /// The backend the last frame was drawn with, or the embedded face when the
-  /// host installed none. The renderer's own wrapper while a frame is in flight,
-  /// so a caller reads this after `draw()` to see what the plot was handed.
+  /// The backend this renderer *borrows*: whatever `set_text_backend()` was given,
+  /// or the embedded face when nothing was installed. It is deliberately **not**
+  /// the `AxisFont` wrapper the renderer builds around it -- after a `draw()`,
+  /// `Plot::text_backend()` reads back that wrapper, because the wrapper is what
+  /// the plot measured with -- so a caller checking or re-pointing the face itself
+  /// asks here.
   const render::TextBackend* text_backend() const noexcept { return font_; }
 
   /// Starts a frame: records `fb_w` x `fb_h` for the projection, flips