fix(rust): normalize setext headings to ATX

[jaredhowland]

Potentially addresses issue #5

New normalization rule for setext headings:

• Added rule 34 (not that one): "Normalize setext headings to ATX headings" (converts headings using ===/--- underlines to #/## ATX style). This is reflected in the rule list, configuration documentation, and sample config generation. [1] [2] [3] [4] [5] [6] [7]

Implementation in code:

• Introduced is_setext_heading and normalize_setext_to_atx_headings functions to detect and convert setext headings during file processing. The conversion logic is integrated into the main processing loop, respecting skip rules. [1] [2]

Testing:

• Added comprehensive tests to ensure setext headings are converted correctly, that horizontal rules are not misinterpreted, and that the rule can be disabled via configuration.

Trade-offs:

• We intentionally normalize all setext headings to ATX in output. This is a formatting choice that reduces parser ambiguity and yields more predictable downstream rule application, at the cost of not preserving original setext style.
• Detection is conservative (skips list items, blockquotes, existing headlines, code fences, and blank lines) to avoid broad behavioral changes.

This pull request was aided by Copilot AI but hand-checked.

[Copilot]

Pull request overview

Adds a new Rust-side normalization rule to convert Setext-style headings (=== / --- underlines) into ATX headings (# / ##), addressing the Setext/horizontal-rule ambiguity that contributed to issue #5.

Changes:

• Added rule 34 (setext-to-atx) to the Rust rule registry and integrated Setext-to-ATX conversion into the main per-line processing pipeline.
• Updated generated config output and README rule documentation to include the new rule.
• Added Rust tests covering Setext conversion, horizontal-rule non-conversion, and configurability via skipping rule 34.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
rust/src/main.rs	Implements rule 34, wires it into processing, updates config init output, and adds tests.
README.md	Updates rule count and documents rule 34 in the rules and configuration sections.

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[ttscoff]

Thanks for this!

[ttscoff]

The test for this did fail, fixing it in post.