feat(render): anti-aliased embedded Adwaita Mono atlas, generated and committed

include/plot/render/text.hpp

@@ -108,6 +108,18 @@ class TextBackend {
   /// text bottoms instead of cell bottoms.
   virtual f32 ascent() const = 0;
 
+  /// The face's em size in font pixels: the size the atlas was baked at, in the
+  /// same units as `height()` and `ascent()`.
+  ///
+  /// It exists so a run can be drawn at the size a caller asked for whatever the
+  /// backend was baked at: the renderer scales by `requested_px / em_px()`, so a
+  /// backend re-baked at another size -- or swapped for a different face -- keeps
+  /// drawing `Style::axis_font_px`, and the renderer never has to know which font
+  /// it holds. A backend with no meaningful em (one whose "font pixels" are an
+  /// arbitrary unit) returns something proportional, `height()` being the
+  /// natural choice.
+  virtual f32 em_px() const = 0;
+
   /// The glyph for `codepoint`, or null when this backend has none -- the caller
   /// then draws nothing for it (a backend that wants a visible missing-glyph box
   /// publishes one as the glyph for the codepoints it cannot render). The