feat(imgui): embeddable plot widget and demo

adapters/imgui/CMakeLists.txt

@@ -0,0 +1,54 @@
+# The Dear ImGui adapter: a widget that draws a plot into a child region of the
+# framebuffer ImGui is rendering its own chrome into, and feeds that plot
+# ImGui's mouse and keyboard state.
+#
+# ImGui is not a packaging dependency here: it is fetched pinned to a release
+# tag (never a branch, so a build is reproducible) and only the sources this
+# adapter needs are compiled -- the core plus the GLFW and OpenGL 3 backends.
+# The fetched sources are third-party code: they are not held to this project's
+# warning set, and the adapter and its example are.
+include(FetchContent)
+FetchContent_Declare(imgui
+  GIT_REPOSITORY https://github.com/ocornut/imgui.git
+  GIT_TAG v1.91.9b
+  GIT_SHALLOW TRUE)
+# imgui has no top-level CMakeLists.txt, so MakeAvailable populates it and stops;
+# the sources are added to the target below by name.
+FetchContent_MakeAvailable(imgui)
+
+if(NOT imgui_SOURCE_DIR OR NOT EXISTS "${imgui_SOURCE_DIR}/imgui.cpp")
+  message(FATAL_ERROR
+    "plot: the Dear ImGui sources for tag v1.91.9b are missing from "
+    "'${imgui_SOURCE_DIR}'. The FetchContent clone failed, so PLOT_BUILD_IMGUI "
+    "cannot link anything; either restore network access or pre-populate the "
+    "source with -DFETCHCONTENT_SOURCE_DIR_IMGUI=<path>.")
+endif()
+
+add_library(imgui STATIC
+  "${imgui_SOURCE_DIR}/imgui.cpp"
+  "${imgui_SOURCE_DIR}/imgui_draw.cpp"
+  "${imgui_SOURCE_DIR}/imgui_tables.cpp"
+  "${imgui_SOURCE_DIR}/imgui_widgets.cpp"
+  "${imgui_SOURCE_DIR}/backends/imgui_impl_glfw.cpp"
+  "${imgui_SOURCE_DIR}/backends/imgui_impl_opengl3.cpp")
+# Both include roots are public: a consumer of the backends -- the adapter and
+# the example -- reaches them as <imgui.h> and <backends/imgui_impl_glfw.h>.
+target_include_directories(imgui PUBLIC
+  "${imgui_SOURCE_DIR}"
+  "${imgui_SOURCE_DIR}/backends")
+
+# imgui_impl_glfw.cpp needs GLFW; a build that also enables the GLFW adapter
+# gets the same target, never a second copy.
+plot_find_glfw(plot_imgui_glfw_dep)
+target_link_libraries(imgui PUBLIC ${plot_imgui_glfw_dep})
+target_compile_features(imgui PUBLIC cxx_std_20)
+
+add_library(plot_imgui STATIC imgui_plot.cpp)
+add_library(plot::imgui ALIAS plot_imgui)
+# The public include root is `adapters/`, so the header is reached as
+# <imgui/imgui_plot.hpp> -- the toolkit's name first, like every other adapter's.
+target_include_directories(plot_imgui PUBLIC "${CMAKE_CURRENT_SOURCE_DIR}/..")
+# ImGui stays PRIVATE: the adapter's header includes no ImGui header, so a
+# consumer of plot::imgui does not inherit the toolkit's headers.
+target_link_libraries(plot_imgui PUBLIC plot::core PRIVATE imgui ${plot_imgui_glfw_dep})
+plot_apply_warnings(plot_imgui)