FetchContent population reuse semantics when re-entering a superbuild from a different directory scope
Context FetchContent is documented to populate external sources once per build tree, caching the result under the build directory and reusing it on subsequent configures unless the declared URL, URL_HASH, or content changes. This contract works predictably for a single top-level configure run. Ambiguity When a superbuild re-enters its own CMake logic from a