security(enrich): AST-validate LLM output before writing to disk (F-9 follow-up)

[gibbon]

v0.2.0 ships F-9 partial mitigation (banner + git-diff workflow). Future hardening: parse the LLM output as the target language and reject if it doesn't match the stub's class/function signature. Per-language parser plumbing is a sizable feature. See F-9.