ratex-parser has unbounded parser recursion that leads to stack overflow (process abort)
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.
