feat: handle mermaid in documentation

[this-is-tobi]

Description

Add support for Mermaid diagrams in built documentation by integrating vitepress-plugin-mermaid.

The withMermaid() wrapper was previously applied during config generation, causing its Vite plugin and markdown-it extensions (which are functions) to be silently dropped by JSON.stringify. The fix moves the withMermaid() call into the generated .vitepress/config.ts so it is evaluated at VitePress runtime, making Mermaid diagrams fully functional in built docs.

Additionally, pre-existing type errors in the test suite were fixed: readdirSync mocks were casting to Dirent<Buffer<ArrayBufferLike>>[], which is incompatible with newer @types/node where the type is Dirent<NonSharedBuffer>[]. All casts were replaced with as any.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

The full test suite (214 tests across 14 files) passes locally. TypeScript compilation (tsc) and Vite build both succeed without errors or warnings.

Test Configuration:

• Node.js: >=20
• Package manager: pnpm@10.32.0
• Toolchain: vitest@4.0.18, typescript@5.9.3, vite@7.3.1

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules
• I have checked my code and corrected any misspellings

[github-actions]

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