Add project files.

tests/editor_headless/README.md

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+# Headless editor logic tests
+
+Runs the editor's real command / picking / camera / tool code (`editor.cpp`, `viewport.cpp`, `render.cpp`)
+against **no-op raylib and Dear ImGui back-ends** (`stubs/` + `impl.cpp`), driving it with a scripted mouse.
+No GPU or window needed, so it also works in CI.
+
+    cmake -B build -DHPP_BUILD_EDITOR_LOGIC_TESTS=ON && cmake --build build --target test_editor_logic
+    ctest --test-dir build -R editor_logic --output-on-failure
+
+Linux + GNU ld/lld only (it links with `--unresolved-symbols=ignore-all` so UI-only code paths may stay
+unresolved). The stubs mirror the raylib 5.5 / ImGui 1.91 signatures the editor uses; they are not a
+re-implementation - they only exist so this code can be compiled and exercised without the real libraries.