engine: add the node graph family to the C ABI facade

- project node enumeration, type/name/label metadata, input
  introspection (id, mapped value type, connection state)
- typed parameter read/write for the eight common NodeValue kinds
  (int/float/bool/rational/color/vec2-4/combo/string), undoable via
  NodeParamSetSplitStandardValueCommand - the split-track value path,
  chosen after proving the standard-value command stores whole
  variants into a single component track
- graph operations (add/remove/connect/disconnect) reusing the
  engine's undo commands, all covered by undo/redo assertions

engine/CMakeLists.txt

@@ -301,4 +301,9 @@ if (BUILD_TESTS)
     if (TARGET olive-render-worker)
         add_dependencies(oakengine_export_test olive-render-worker)
     endif ()
+
+    make_oakengine_test(oakengine_node_test)
+    target_compile_definitions(oakengine_node_test PRIVATE
+            OAK_TEST_SOURCE_DIR="${CMAKE_SOURCE_DIR}"
+    )
 endif ()