fast-grep-rust

Use fast-grep (`fgr`) for regex and literal text search across this repository. When an index exists it is dramatically faster than `grep`/`ripgrep`; without an index it is comparable to ripgrep. TRIGGER when: searching for code (function, symbol, identifier, string), counting occurrences, or listing files matching a pattern in this repo. SKIP for: git-history search (use `git log -G`/`git grep`), binary files, or patterns that require lookaround / backreferences (the Rust `regex` crate does not support them — fall back to `rg -P` or `grep -P`).