Add StructEval structured-output benchmark

[reacher-z]

Summary

• add the TMLR StructEval paper to the Benchmark section
• link the official paper, repository, project page, and dataset
• surface its evaluation of generation and conversion across 2,035 examples, 18 text and renderable formats, and 44 task types

Validation

• git diff --check
• verified the entry metadata against the published paper and official project page
• verified all four official links; the project page loaded through browser retrieval after local curl resets
• npx --yes awesome-lint remains red on the repository-wide heading, TOC, and established [[Label]] link-style baseline; this entry follows that existing format

Disclosure

I am a StructEval maintainer and am submitting this entry transparently for maintainer review.

This contribution was prepared with assistance from OpenAI Codex.