ratex-parser panics on `\verb` with a multibyte delimiter (UTF-8 byte-boundary slice)

crates/ratex-parser/src/parser.rs, parse_symbol_inner:

 1if let Some(stripped) = text.strip_prefix("\\verb") {       // parser.rs:901
 2    self.consume();
 3    let arg = stripped.to_string();                         // e.g. "éxé"
 4    let star = arg.starts_with('*');
 5    let arg = if star { &arg[1..] } else { &arg };          // parser.rs:905  (also byte-sliced)
 6    if arg.len() < 2 {                                      // byte length
 7        return Err(ParseError::new("\\verb assertion failed", Some(&nucleus)));
 8    }
 9    let body = arg[1..arg.len() - 1].to_string();           // parser.rs:910  <-- PANIC on multibyte delimiter
10    ...
11}

For input \verbéxé: arg = "éxé", where é = U+00E9 (bytes C3 A9). arg.len() is the byte length (5), the < 2 guard passes, and arg[1..4] starts at byte index 1 — inside the first é (bytes 0..2) — so the slice panics. The lexer groups \verb<delim>…<delim> correctly with char semantics (lexer.rs lex_verb); only the parser mishandles it.

PoC

 1$ printf '\\verb\xc3\xa9x\xc3\xa9\n' | ./target/release/parse
 2thread 'main' panicked at crates/ratex-parser/src/parser.rs:910:27:
 3start byte index 1 is not a char boundary; it is inside 'é' (bytes 0..2 of string)
 4Aborted (core dumped)            # exit 134 — panic=abort kills the whole process

Impact

Any application that renders untrusted LaTeX through RaTeX (web “render this math” endpoint, WASM in-browser use, the FFI embedded in another app) can be crashed by a tiny string. With panic = "abort" in release builds, the crash takes down the whole process / server, so a single malicious formula causes a full-service DoS (and, in batch pipelines, drops all queued work).

Slice by character boundaries instead of byte indices, mirroring the UTF-8-correct logic the lexer already uses. For example:

 1let chars: Vec<char> = arg.chars().collect();
 2if chars.len() < 2 { return Err(ParseError::new("\\verb assertion failed", Some(&nucleus))); }
 3let body: String = chars[1..chars.len() - 1].iter().collect();

(Apply the same char-aware handling to the * strip at parser.rs:905.) More broadly, consider not using panic = "abort" for builds embedded in long-running services, and/or wrapping parsing in catch_unwind at the FFI/WASM boundary — but the byte-slice fix is the direct correction.