fix(render): environment-aware atlas drift guard, pinned text scale, shipped-atlas coverage

include/plot/render/text.hpp

@@ -115,9 +115,11 @@ class TextBackend {
   /// 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.
+  ///
+  /// A backend that *wraps* another to draw it at a size scales this with the
+  /// rest of its metrics -- it is in the wrapper's units, like `height()` -- so a
+  /// caller that reads the backend back and draws at `requested_px / em_px()`
+  /// gets a ratio of two screen-pixel quantities and cannot scale twice.
   virtual f32 em_px() const = 0;
 
   /// The glyph for `codepoint`, or null when this backend has none -- the caller