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

adapters/qt6/CMakeLists.txt

@@ -19,9 +19,19 @@ endif()
 
 add_library(plot_qt6 STATIC qt_plot_widget.cpp qt_plot_widget.hpp)
 add_library(plot::qt6 ALIAS plot_qt6)
+# The installed tree names it plot::qt6, like the build tree (see plot_core's).
+set_target_properties(plot_qt6 PROPERTIES EXPORT_NAME qt6)
+
 # The public include root is `adapters/`, so the header is reached as
 # <qt6/qt_plot_widget.hpp> -- the toolkit's name first, like every other adapter's.
-target_include_directories(plot_qt6 PUBLIC "${CMAKE_CURRENT_SOURCE_DIR}/..")
+# The build interface is that root; the install one is the prefix's `include`,
+# where both headers are installed as <qt6/...> -- the same spelling. The pointer
+# anchor in qt_pointer.hpp is internal API, but it still ships: the widget's header
+# includes it, so a consumer that gets one without the other does not compile.
+target_include_directories(plot_qt6 PUBLIC
+  $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/..>
+  $<INSTALL_INTERFACE:include>)
+
 # Qt is PUBLIC, unlike GLFW and ImGui: the widget's header derives from
 # QOpenGLWidget, so a consumer needs Qt's headers and libraries to use it at all.
 target_link_libraries(plot_qt6 PUBLIC
@@ -35,3 +45,12 @@ target_link_libraries(plot_qt6 PUBLIC
 # Qt class in it, and AUTOMOC scans every source of a target it is set on.
 set_target_properties(plot_qt6 PROPERTIES AUTOMOC ON)
 plot_apply_warnings(plot_qt6)
+
+# Qt is a system SDK found with find_package() -- a missing Qt6 is a configure
+# error above, never a fetch -- so the installed config file can re-find it with
+# the same components and the widget's public Qt link interface is satisfiable.
+plot_record_adapter_export(plot_qt6 TRUE "Qt6 was resolved by find_package"
+  "find_dependency(Qt6 COMPONENTS Core Gui Widgets OpenGLWidgets)\n")
+if(PLOT_INSTALL)
+  install(FILES qt_plot_widget.hpp qt_pointer.hpp DESTINATION include/qt6)
+endif()