[Yuna-Celisse]

Summary

Closes #227.

• Runs the repository's Markdownlint rules in the browser editor, including the archived-directory overrides.
• Highlights affected editor lines and lists rule, line, and repair guidance before submit.
• Rejects unclosed code fences/container blocks and validates Mermaid syntax before submitting.
• Renders valid mermaid / mmd fenced blocks as SVG in the in-page preview and replaces invalid diagrams with an actionable error card.
• Documents the new checker and Mermaid preview in the writing guide.

Scope

• Tutorial
• Process
• Repair
• Archived
• Navigation
• Assets or templates
• CI or tooling
• Governance docs

Checklist

• I updated the relevant sidebar or confirmed no navigation change is needed.
• I checked internal links and asset paths affected by this PR.
• I kept historical archived content unchanged unless this PR is explicitly about archive maintenance.
• I documented follow-up work that is intentionally outside this PR.

Verification

• pnpm run ci:lint:markdown
• pnpm run ci:lint:eslint
• pnpm docs:build
• Focused validation tests: 8 passed
• Remaining non-Windows-sensitive suite: 157 passed
• Browser-tested editor errors, inline highlights, valid Mermaid SVG, invalid Mermaid card, and archived config behavior

Notes

This Windows checkout has pre-existing line-ending-sensitive failures in utils/docs-api.test.ts and checks/site.test.ts; the full local run was 171 passed / 3 failed. ci:verify also reports 42 existing path-separator anchor mismatches on Windows. The individual lint commands and production build pass, and Linux CI remains the authoritative check.

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