fix(imgui): keep the plot inside the child's clip rect and reject unusable input

The widget drew a region derived from the child's content rect alone, so a child
the hosting window cut off cleared and painted over the host's pixels; it also
scaled ImGui's (-FLT_MAX, -FLT_MAX) no-pointer sentinel and differenced it against
the last real position, turning a drag out of the window into a ~1e35 pan, and it
forgot the host's button levels across a re-bind, handing the next plot a press
that never happened.

The region is now the content rect intersected with the child's clip rect (a
no-op when nothing clips it), an invalid position is no position update, and the
button levels survive a re-bind. The controller sanitises non-finite coordinates
and deltas for any host, and the widget's input translation now has a committed
test on the headless EGL context.

CMakeLists.txt

@@ -34,21 +34,13 @@ target_compile_features(plot_core PUBLIC cxx_std_20)
 target_link_libraries(plot_core PRIVATE ${CMAKE_DL_LIBS})
 plot_apply_warnings(plot_core)
 
-if(PLOT_BUILD_TESTS)
-  # Test-only: counts the pyramid's covering-decomposition entries and its bucket
-  # merges so a test can pin the query's O(log n) cost and the append path's
-  # amortised O(1) cost, neither of which any answer reveals. Off in every
-  # non-test build (see include/plot/data/detail/{cover,merge}_count.hpp).
-  target_compile_definitions(plot_core PRIVATE PLOT_PYRAMID_COUNT_COVERS
-                                             PLOT_PYRAMID_COUNT_MERGES)
-
-  enable_testing()
-  add_subdirectory(tests)
-endif()
-
 # Adapters own the surface and translate their host's events; examples link the
-# adapter they demo. Both are opt-in, so the core library and its tests still
-# build on a bare toolchain.
+# adapter they demo. Both are opt-in, so the core library and its tests still build
+# on a bare toolchain.
+#
+# They are added before the test suite on purpose: a test that drives an adapter
+# needs that adapter's targets to exist, and with both switches on the suite can
+# then exercise them (see tests/CMakeLists.txt).
 if(PLOT_BUILD_GLFW)
   add_subdirectory(adapters/glfw)
 endif()
@@ -60,3 +52,15 @@ endif()
 if(PLOT_BUILD_EXAMPLES)
   add_subdirectory(examples)
 endif()
+
+if(PLOT_BUILD_TESTS)
+  # Test-only: counts the pyramid's covering-decomposition entries and its bucket
+  # merges so a test can pin the query's O(log n) cost and the append path's
+  # amortised O(1) cost, neither of which any answer reveals. Off in every
+  # non-test build (see include/plot/data/detail/{cover,merge}_count.hpp).
+  target_compile_definitions(plot_core PRIVATE PLOT_PYRAMID_COUNT_COVERS
+                                             PLOT_PYRAMID_COUNT_MERGES)
+
+  enable_testing()
+  add_subdirectory(tests)
+endif()