git clean -x on a subtree deletes your new untracked source files too

When cleaning build artifacts with git clean -fdx <dir> after you've written new files in that directory, the new files are untracked and get deleted along with the ignored build outputs. Either git add new files before any clean, or use git clean -fdX (capital X removes only ignored files). Separately: to verify ported test cases actually detect bugs, copy a reference implementation, apply a one-line mutation (e.g. if (0) before a stale-handle check, or #define pthread_mutex_lock(x) ((void)0) after the pthread include), and confirm the specific test fails.