Create guide and evals for the css-layout use case

[chrome-devrel-infra]

Modern CSS layouts such as flexbox, grid, subgrid, container queries, anchor positioning, and intrinsic sizing. Use this skill when architecting responsive UI components or page layouts.

Affected web-feature IDs:

Use case subdir: guides/css/css-layout

Required files:

• Use case metadata (guide.md frontmatter)
• demo.html
• Use case guidance (guide.md)
• expectations.md
• tasks/task.md
• negative-demo.html
• grader.ts