This updates our synthetic generator for UTF-8 to expose: - Flags to change 1/2/3/4-byte UTF-8 character distribution - Flags to have only printable characters so we can benchmark pure UTF-8 vs our control sequence finder. - Flags to have invalid characters so we can benchmark our error handling. This also adds an AGENTS.md to src/benchmark so agents can do the right thing more easily. These are necessary to robustly benchmark our libc++ removal PR.