NOT a stable public API. Engine-tier @@/ sigil-grammar parser —
published so an independent-posture client (CCC, RFC_202607180403) can
consume it from crates.io, and so the ppoppo clients (CCC + CWC) share ONE
grammar source. The @@/ grammar co-evolves with the chat surface; do not
build a 3rd-party integration on this crate's API expecting stability.