Module scripts/validators/article/rules/mermaid-fences

Description

Mermaid fence integrity helpers — detect unclosed ```mermaid fences and count opening fences for coverage checks.

         Rule census: extracted from
         `scripts/validate-article.ts` lines 250–313
         (`UnclosedMermaidFence`, `findUnclosedMermaidFences`,
         `countMermaidOpenings`). Logic is byte-identical to the
         original.

@author Hack23 AB @license Apache-2.0