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

CMakeLists.txt

@@ -43,6 +43,33 @@ target_compile_features(plot_core PUBLIC cxx_std_20)
 target_link_libraries(plot_core PRIVATE ${CMAKE_DL_LIBS})
 plot_apply_warnings(plot_core)
 
+# The optional runtime-font backend: plot::text::TrueTypeFont, which bakes a real
+# TrueType/OpenType file into a fixed-cell atlas at run time (see
+# include/plot/text/true_type_font.hpp). It is the one target that compiles the
+# vendored third_party/stb_truetype.h, so it is opt-in and separable -- the core
+# library, its public headers and its tests build without it -- and its default
+# follows the vendored header's presence: a tree that has the file wants the
+# feature, and one that does not cannot have it. Asking for it explicitly without
+# the header is an error rather than a silent downgrade, because the option is
+# then the only thing that would have said so.
+if(EXISTS "${CMAKE_CURRENT_SOURCE_DIR}/third_party/stb_truetype.h")
+  set(PLOT_VENDORED_STB_TRUETYPE TRUE)
+else()
+  set(PLOT_VENDORED_STB_TRUETYPE FALSE)
+endif()
+option(PLOT_BUILD_TTF_FONT
+  "Build the optional plot::text::TrueTypeFont backend (vendored stb_truetype)"
+  ${PLOT_VENDORED_STB_TRUETYPE})
+if(PLOT_BUILD_TTF_FONT AND NOT PLOT_VENDORED_STB_TRUETYPE)
+  message(FATAL_ERROR
+    "plot: PLOT_BUILD_TTF_FONT=ON needs third_party/stb_truetype.h, which this tree does "
+    "not have. Restore it -- include/plot/text/true_type_font.hpp records the version and "
+    "the source URL it came from -- or configure with -DPLOT_BUILD_TTF_FONT=OFF.")
+endif()
+if(PLOT_BUILD_TTF_FONT)
+  add_subdirectory(src/text)
+endif()
+
 # Adapters own the surface and translate their host's events; examples link the
 # adapter they demo. Both are opt-in, so the core library and its tests still build
 # on a bare toolchain.
@@ -120,7 +147,15 @@ if(PLOT_INSTALL)
     ARCHIVE DESTINATION lib
     LIBRARY DESTINATION lib
     RUNTIME DESTINATION bin)
-  install(DIRECTORY include/plot DESTINATION include)
+  # The optional backend's header goes in only when that backend was built: an
+  # installed header with no library behind it is a promise the prefix cannot keep,
+  # and which configuration a prefix came from is not something a consumer can see.
+  if(PLOT_BUILD_TTF_FONT)
+    install(DIRECTORY include/plot DESTINATION include)
+  else()
+    install(DIRECTORY include/plot DESTINATION include
+      PATTERN "true_type_font.hpp" EXCLUDE)
+  endif()
 
   install(EXPORT plotTargets
     NAMESPACE plot::