Lint rule: heading level skips

[roxspring]

Implement a lint rule that reports when heading levels skip a level (e.g. H1 → H3 without H2).

Scope

• Walk all Heading nodes in document order
• Report a violation if the heading level increases by more than 1 from the previous heading
• Report file path and line number

Example violation

guide/editor.md:12: heading level skip — H1 followed by H3 (expected H2)

Acceptance criteria

• H1 → H2 → H3: no violation
• H1 → H3: violation reported
• H1 → H2 → H1 → H2: no violation (level can decrease freely)
• Document with no headings: no violation