Squashed onto current main. Adds src/engine/common/Assert.h (EV_PARAM_CHECK / EV_ASSERT backed by zeroerr), an EVENGINE_ENABLE_ASSERTS option (non-Debug builds strip ZEROERR_NO_ASSERT unless enabled), parameter checks at key entry points, Doxygen comments across headers, Doxyfile + make docs / CMake docs targets, and Windows setup + code conventions in AGENTS.md.