Kernel SDK 1.1 migration preview

Standalone C headers, CMake configuration and manifest schema for ABI v1 plugins, optional RMD1 model descriptions and CPE1 native CPU arithmetic. Install by extracting the archive and setting CMAKE_PREFIX_PATH to its top-level directory. Source: 4b4ded0. Phase 2 complete backend extraction remains in progress; this is an SDK preview, not a declaration of Phase 2 completion.