Harden the OpenQASM 2.0 parser against malformed input

[mizuta-c5]

Harden the OpenQASM 2.0 parser against malformed input

Summary

The qasmtools recursive-descent parser can be made to abort or hang the host
process on malformed OpenQASM input reachable through the public entry point
parse_string (and therefore pystaq.parse_str, the staq CLI, and any project
embedding the header-only parser). This PR closes five such cases without changing
behaviour on valid input.

Input	Before	Cause
integer literal > INT_MAX	abort()	uncaught std::out_of_range from std::stoi
real literal out of float range (1e99999999)	abort()	uncaught std::out_of_range from std::stof
deeply nested (((…))) in an expression	SIGSEGV	unbounded recursion in parse_exp/parse_atom
unterminated string at EOF (include "…)	infinite loop	lex_string loop condition omits EOF
wrong token where an id/int is expected (qreg 5[2];)	abort()	std::bad_variant_access from as_string()/as_int() on a mismatched token

The first, second and fifth cases matter beyond a CLI: they raise C++ exceptions
that are not the parser's documented ParseError, so a caller catching only
ParseError still aborts. The recursion and infinite-loop cases bypass exception
handling entirely (including pybind11's translation), so a pystaq-based service
crashes or hangs too.

Changes

• lexer.hpp — wrap std::stoi/std::stof in try/catch (std::out_of_range)
  and return a Token::Kind::error (mirroring the existing unmatched-quote path);
  add && buf_->peek() != EOF to the lex_string loop (the same guard already used
  by the comment lexer).
• parser.hpp — bound expression-parsing recursion with an RAII depth guard
  (MAX_EXPR_DEPTH = 1000); on overflow, set the error flag and raise ParseError.
• token.hpp — make as_int/as_real/as_string total using std::get_if,
  returning a default on a kind/value mismatch instead of throwing. The parser's
  error_ flag is set independently, so malformed input still ends in ParseError.
• CHANGES.md — one line under # Pre-release.

Testing

• Each of the five inputs above now returns gracefully (ParseError or a clean
  parse) instead of aborting/hanging.
• All 82 sample .qasm files under misc/ and pystaq/ parse identically before
  and after this change — no behavioural change on valid input.

Total diff: 4 files, +49/-9.