feat(text): optional vendored stb_truetype backend for runtime external fonts

include/plot/render/embedded_mono_font.hpp

@@ -8,11 +8,11 @@ namespace plot::render {
 /// The renderer's built-in text backend: an anti-aliased greyscale atlas of
 /// Adwaita Mono, baked at authoring time and committed as data.
 ///
-/// There is no font file, FreeType or stb_truetype behind this: the coverage
-/// bytes and the glyph table are generated by `tools/fonts/gen_atlas.py` into
-/// `src/render/embedded_mono_font.cpp`, so `plot_core` stays dependency-free and
-/// nothing rasterises at build or run time. That file carries the provenance --
-/// source face, its version, the em size, the supersample factor and the
+/// There is no font file, FreeType or third-party rasteriser behind this: the
+/// coverage bytes and the glyph table are generated by `tools/fonts/gen_atlas.py`
+/// into `src/render/embedded_mono_font.cpp`, so `plot_core` stays dependency-free
+/// and nothing rasterises at build or run time. That file carries the provenance
+/// -- source face, its version, the em size, the supersample factor and the
 /// coverage set -- and `gen_atlas.py --check` fails if it has drifted from its
 /// input. Adwaita Mono is SIL OFL 1.1 (a copy is committed at
 /// `licences/OFL-1.1.txt`); the class is named neutrally because OFL reserves
@@ -27,6 +27,12 @@ namespace plot::render {
 ///
 /// A codepoint outside the coverage set draws the baked fallback box, so a label
 /// the atlas cannot spell is visible rather than silently blank.
+///
+/// This is the only backend the core library ships. The one that bakes a font
+/// file at run time belongs to the optional target -- `plot/text/true_type_font.hpp`,
+/// deliberately not part of `include/plot/plot.hpp` -- and no public header
+/// outside `plot/text/` names what rasterises it, which is the isolation that
+/// target exists to keep.
 class EmbeddedMonoFont final : public TextBackend {
  public:
   /// The size the atlas was baked at, in font pixels, and the coverage samples