GHSA-4w5h-hx6r-28q7

### Summary

RaTeX’s recursive-descent parser recurses one (or more) native stack frame per nesting level at `{`, `\left`, `\sqrt{`, `^{`, etc, with **no maximum depth limit**. A short, ~10 KB input of nested groups overflows the 8 MB main-thread stack and aborts the process. With `panic = "abort"` (`Cargo.toml:48`), and because a Rust stack overflow is always a fatal `SIGABRT` regardless of panic strategy this is an unrecoverable, whole-process denial of service reachable from a single untrusted LaTeX string.

### Details

The mutual recursion has no depth guard (`crates/ratex-parser/src/parser.rs`):

``` parse_expression (:113) -> parse_atom (:281/285) -> parse_group (:451) ^ | | on '{' (:459) recurse | +--------------------------+ ```

`\left` adds another recursive edge: `handle_left` → `parse_expression` (`crates/ratex-parser/src/functions/left_right.rs:47`). The only counters present are unrelated to depth: `leftright_depth` (a `\right`-matching counter, `parser.rs:24`) and the macro expander’s `max_expand = 1000` (`macro_expander.rs:64`), which does **not** gate brace / `\left` recursion (those tokens never pass through `expand_once`). There is no `recursion_limit`/depth parameter on `parse_group`, `parse_expression`, or `parse_atom`.

### PoC

<img width="1097" height="158" alt="image" src="https://github.com/user-attachments/assets/29b837a2-c455-4cb6-a055-514b31c999c6" />

``` $ python3 -c 'import sys;sys.stdout.write("{"*200000+"x"+"}"*200000)' | ./target/release/parse thread 'main' has overflowed its stack fatal runtime error: stack overflow, aborting Aborted (core dumped) # exit 134 ```

(Other nesting forms work equally, e.g. `\left(`×N, `\sqrt{`×N, `^{`×N.)

### Impact

A single small request crashes the whole RaTeX process. In a typical server-side math-rendering service this is a reliable, unauthenticated DoS; on smaller worker-thread stacks (e.g. a 512 KB async runtime thread) only a few hundred bytes of nesting are required.