feat(build): install/export rules, umbrella header, verified consumer

adapters/glfw/CMakeLists.txt

@@ -8,10 +8,36 @@ plot_find_glfw(plot_glfw_dep)
 
 add_library(plot_glfw STATIC glfw_host.cpp)
 add_library(plot::glfw ALIAS plot_glfw)
+# The installed tree names it plot::glfw, like the build tree (see plot_core's).
+set_target_properties(plot_glfw PROPERTIES EXPORT_NAME glfw)
+
 # The public include root is `adapters/`, so the header is reached as
 # <glfw/glfw_host.hpp> -- the toolkit's name first, like every other adapter's.
-target_include_directories(plot_glfw PUBLIC "${CMAKE_CURRENT_SOURCE_DIR}/..")
+# The build interface is that root; the install one is the prefix's `include`,
+# where glfw_host.hpp is installed as <glfw/glfw_host.hpp>, the same spelling.
+target_include_directories(plot_glfw PUBLIC
+  $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/..>
+  $<INSTALL_INTERFACE:include>)
+
 # GLFW stays PRIVATE: the header forward-declares GLFWwindow and includes no GLFW
 # header, so a consumer of plot::glfw does not inherit the toolkit.
 target_link_libraries(plot_glfw PUBLIC plot::core PRIVATE ${plot_glfw_dep})
 plot_apply_warnings(plot_glfw)
+
+# PLOT_GLFW_DEP is either a target find_package()/pkg-config created or one
+# FetchContent built. The first two can be promised to a consumer -- the installed
+# config file re-runs the very same discovery -- but a fetched GLFW cannot: there
+# is no glfw3Config.cmake on the consumer's machine to find, so exporting this
+# adapter would hand out a target its link interface cannot satisfy. In that case
+# the adapter is not installed at all.
+plot_glfw_dependency_snippet(plot_glfw_dependency)
+if(PLOT_GLFW_DEP_ORIGIN STREQUAL "FetchContent")
+  plot_record_adapter_export(plot_glfw FALSE
+    "GLFW was fetched with FetchContent, so a consumer's find_package(plot) cannot re-find it" "")
+else()
+  plot_record_adapter_export(plot_glfw TRUE
+    "GLFW was resolved by ${PLOT_GLFW_DEP_ORIGIN}" "${plot_glfw_dependency}")
+  if(PLOT_INSTALL)
+    install(FILES glfw_host.hpp DESTINATION include/glfw)
+  endif()
+endif()