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

src/render/renderer.cpp

@@ -3,6 +3,7 @@
 #include <plot/gl/api.hpp>
 #include <plot/gl/caps.hpp>
 #include <plot/gl/loader.hpp>
+#include <plot/render/detail/text_scale.hpp>
 
 #include <algorithm>
 #include <cmath>
@@ -97,15 +98,12 @@ void warn_replaced_text_backend(const render::TextBackend* previous) noexcept {
 
 void Renderer::AxisFont::reset(const render::TextBackend& font, f32 requested_px) noexcept {
   font_ = &font;
-  // The requested size is an em size, exactly as `em_px()` reports the size the
-  // atlas was baked at, so the ratio between them is all the scaling there is:
-  // an anti-aliased face is legible at any ratio, and a style asking for 12 px
-  // from a 13 px bake gets 12/13 rather than a snapped whole multiple. A backend
-  // that cannot say (`em_px()` at or below zero, or NaN) keeps a 1:1 scale rather
-  // than dividing by it.
-  const f32 baked = font.em_px();
-  const f32 ratio = requested_px / baked;
-  scale_ = (baked > 0.0f && ratio > 0.0f) ? ratio : 1.0f;
+  // An anti-aliased face is legible at any ratio, so a style asking for 12 px
+  // from a 13 px bake is drawn at 12/13 rather than snapped to a whole multiple
+  // of the cell. `detail::text_scale` owns that rule, including the 1:1 fallback
+  // for a backend that cannot state an em -- it is one function, and the test
+  // suite asserts it without a GL context.
+  scale_ = render::detail::text_scale(requested_px, font.em_px());
   cell_px_ = font.height() * scale_;
 }
 
@@ -122,9 +120,12 @@ f32 Renderer::AxisFont::ascent() const {
 }
 
 f32 Renderer::AxisFont::em_px() const {
-  // The em is the one metric that does not scale: it is the face's own unit, and
-  // it is what the scale above was computed from.
-  return font_ != nullptr ? font_->em_px() : 0.0f;
+  // Scaled with the rest of the metrics, not forwarded raw: `TextBackend::em_px()`
+  // is documented to be in the same units as `height()` and `ascent()`, and this
+  // wrapper's are all screen pixels. A caller that reads the backend back off the
+  // plot and draws at `requested_px / em_px()` therefore gets 1:1 rather than
+  // scaling a second time (see the base-class contract).
+  return font_ != nullptr ? font_->em_px() * scale_ : 0.0f;
 }
 
 const render::Glyph* Renderer::AxisFont::glyph(u32 codepoint) const {