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

cmake/plotConfig.cmake.in

@@ -0,0 +1,23 @@
+@PACKAGE_INIT@
+
+# Package config for `plot`. Include it with
+#
+#   find_package(plot 0.1 REQUIRED)
+#   target_link_libraries(your_target PRIVATE plot::core)
+#
+# `plot::core` is the library, and carries no third-party dependency at all. The
+# adapter targets (`plot::glfw`, `plot::qt6`, `plot::gtk4`) are here only when
+# their toolkit could be promised to a consumer: an adapter whose toolkit the
+# build fetched with FetchContent is deliberately not installed, because this
+# file has nothing to point a consumer at -- see the build's configure log for
+# which adapters were exported and which were skipped, and why.
+#
+# Those toolkits are re-found below, before plotTargets.cmake is included: the
+# adapter targets name them in their own link interfaces, so the imported targets
+# have to exist first.
+
+include(CMakeFindDependencyMacro)
+@PLOT_ADAPTER_DEPENDENCIES@
+include("${CMAKE_CURRENT_LIST_DIR}/plotTargets.cmake")
+
+check_required_components(plot)