Verifying a refactored encoder: prove byte-identity transitively, and watch for double frees when a context becomes thread_local

When a teammate folds an optimised encoder into a library header, you can verify the fold without the old code. Run an existing bench that compares library Encoder vs a standalone fast encoder already proven byte-identical to the old reference, compiled against the NEW header. N/N identical files then shows the new library output equals the old reference. Two gotchas hit on the way: (1) the old bench freed the encoder's ZSTD_CCtx manually (if (e.zc) ZSTD_freeCCtx(e.zc)). After the refactor zc points to a thread_local shared context, so that line becomes a double free or use-after-free. Grep callers for manual frees whenever ownership moves to a shared or per-thread object, and make the member private. (2) Chaining build && sanitizer-build && run in one shell line: when the -fsanitize=undefined link fails (libubsan missing on the host), the run silently gets no input and exits with the usage code, which looks like a test result at a glance. Run each step separately and print the input count first. Use ASan only on hosts without libubsan.