feat(imgui): embeddable plot widget and demo

cmake/plot_options.cmake

@@ -31,3 +31,53 @@ function(plot_apply_warnings target)
       -Wshadow)
   endif()
 endfunction()
+
+# Resolves GLFW once for every target that needs it -- the GLFW host adapter and
+# the Dear ImGui backend both do. Discovery follows the design spec's order:
+# find_package first, pkg-config next, FetchContent only when neither can supply
+# GLFW (the last resort, because it needs the network). The result is cached in
+# PLOT_GLFW_DEP, so a build with several adapters enabled discovers -- and at
+# worst fetches -- exactly one copy. Sets <out_var> to the target to link.
+function(plot_find_glfw out_var)
+  if(NOT DEFINED PLOT_GLFW_DEP)
+    set(plot_glfw_dep "")
+
+    find_package(glfw3 3.3 QUIET)
+    if(TARGET glfw)
+      set(plot_glfw_dep glfw)
+    elseif(TARGET glfw3::glfw3)
+      set(plot_glfw_dep glfw3::glfw3)
+    endif()
+
+    if(NOT plot_glfw_dep)
+      find_package(PkgConfig QUIET)
+      if(PkgConfig_FOUND)
+        pkg_check_modules(GLFW_PC QUIET IMPORTED_TARGET glfw3)
+        if(TARGET PkgConfig::GLFW_PC)
+          set(plot_glfw_dep PkgConfig::GLFW_PC)
+        endif()
+      endif()
+    endif()
+
+    if(NOT plot_glfw_dep)
+      message(STATUS "plot: glfw3 not found via find_package or pkg-config; fetching it")
+      include(FetchContent)
+      set(GLFW_BUILD_DOCS OFF CACHE BOOL "" FORCE)
+      set(GLFW_BUILD_TESTS OFF CACHE BOOL "" FORCE)
+      set(GLFW_BUILD_EXAMPLES OFF CACHE BOOL "" FORCE)
+      set(GLFW_INSTALL OFF CACHE BOOL "" FORCE)
+      FetchContent_Declare(glfw
+        GIT_REPOSITORY https://github.com/glfw/glfw.git
+        GIT_TAG 3.4
+        GIT_SHALLOW TRUE)
+      FetchContent_MakeAvailable(glfw)
+      set(plot_glfw_dep glfw)
+    endif()
+
+    set(PLOT_GLFW_DEP "${plot_glfw_dep}" CACHE INTERNAL
+        "GLFW target the plot adapters link against")
+    message(STATUS "plot: GLFW adapter dependency: ${plot_glfw_dep}")
+  endif()
+
+  set(${out_var} "${PLOT_GLFW_DEP}" PARENT_SCOPE)
+endfunction()