chore: commit fuzz/Cargo.lock for reproducible fuzz builds

fuzz/ is now an independent workspace (excluded from the root workspace
via workspace.exclude).  Committing its Cargo.lock preserves the same
reproducibility guarantee as the root Cargo.lock.

https://claude.ai/code/session_01EJLZKDtcF1RGdKx6TcUTQG

Author: claude