Phase 2 begins: maps, JSON, a reference operator, log levels

stash{ key: expr, .. } is a real String-keyed map; every litter/breed now derives serde::Serialize/Deserialize for JSON (or YAML/CSV) round-trips via serde_json::to_string(&value); a new &expr reference operator unblocks calling real Rust APIs that need one; warn /error join craft as real logging severity levels.