Initial release of secondary-style-perturbation — adds a secondary, subtle style perturbation step for previously rewritten text.

- Further breaks up sentence rhythm and adds short sentences to reduce style consistency.
- Enhances expression variety by introducing synonyms and varied sentence structures.
- Introduces mild local logical jumps, including transitional phrases for academic tone.
- Diversifies use of referential terms for improved naturalness.
- Strict constraints: keeps academic tone, no added info, and minor changes only.