fix: bound plain-text runs in parse_prompt_attention regex

[fszontagh]

Summary

parse_prompt_attention lexes the prompt with an unbounded plain-text alternative. libstdc++'s std::regex recurses once per matched character, so a prompt of a few tens of kilobytes overflows the stack and segfaults before tokenization.

Bound the run length. Splitting a long run is safe because the function's final pass merges adjacent segments of equal weight, and B is excluded from the character class, so a chunk boundary can never fall inside a BREAK.

Related Issue / Discussion

None.

Additional Information

A 45KB prompt exits 139 (SIGSEGV) during tokenization before the change and completes normally after it.

Parsing results are unchanged: comparing old and new output across weighted parentheses, nested ((...)), [...], escaped \(, multiple BREAKs, BREAKING and bare B, bare colons, the empty string, multi-byte UTF-8, and a 21KB run both bare and inside (...:1.3), every case is byte-identical.

Checklist

• I have read and confirmed this PR follows the contribution guidelines.