Build structure

• cmake/WeaveCLLVM.cmake finds LLVM/Clang, sets -fno-rtti/-fno-exceptions to match the LLVM build, and provides weavec_link_llvm / weavec_link_clang which respect LLVM_LINK_LLVM_DYLIB / CLANG_LINK_CLANG_DYLIB.
• cmake/WeaveCHelpers.cmake provides weavec_add_library / weavec_add_executable, which apply warnings, include paths and export metadata uniformly.
• Everything is installed with a CMake package config (find_package(WeaveC)) so external tools can link weavec::Core or weavec::Frontend.