docs: add Three.js section demo

[akkauppi]

What changed

Adds a local Three.js to section-linework to SVG example with checked-in source and section previews. The README now explains the intended audience, practical use cases, origin as a vibe-coded personal-project extraction, and the deliberate scope limits.

The release checks now verify generated fixtures and audit the development dependency graph. Vitest was upgraded to resolve the identified advisories.

Why

The package needed a concrete, public workflow showing how applications choose meshes and interface pairs while the core remains neutral and renderer-free.

Verification

• npm ci
• npm audit --audit-level=high
• npm run typecheck
• npm run typecheck:examples
• npm test
• npm run render:three-section-demo with fixture diff check
• npm run build
• npm run pack:check