Function extractMermaidBlocks

Walk markdown line-by-line and extract every ```mermaid block.

Recognition rules (matching the renderer + article-validator):

• Opening fence: /^```mermaid[\t ]*$/
• Closing fence: /^```[\t ]*$/
• Any other ``` line (e.g. another language tag) is treated as an implicit close and the current block is reported as unclosed.

Pure function — never mutates markdown or any global state.