Avoid backtracking in setext marker detection

[lepture]

Motivation

• The existing SETEXT_MARKER_RE used nested repetition which can exhibit catastrophic backtracking on long near-matches and lead to denial-of-service when processing attacker-controlled input.

Description

• Replace the nested-repetition regex with an equivalent linear-time pattern in src/wenmode/renderers/markdown.py to prevent exponential backtracking.
• Add tests in tests/test_renderers.py to cover spaced and unspaced setext markers and to exercise a long near-match to guard against regex DoS regressions.
• Adjust rendering assertions to verify the expected escaped output and the safe handling of long non-setext markers.

Testing

• Ran the renderer tests with PYTHONPATH=src uv run pytest tests/test_renderers.py -q which passed (106 passed).
• Ran the full test suite with PYTHONPATH=src uv run pytest -q which passed (2303 passed).
• Ran linting and static checks with uv run ruff check src/wenmode/renderers/markdown.py tests/test_renderers.py which reported no issues.
• Ran type checks with PYTHONPATH=src uv run mypy which completed with no reported problems.

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Codex Task