Add CLI to validate layout specifications from YAML

[cyrilschumacher]

Introduce a minimal CLI for flexpect to validate layout rules defined in a YAML file. This enables running layout assertions outside of TypeScript tests and supports CI usage.

Commands

flexpect validate <file>

Run layout validations from a YAML spec.

Options:

• --viewport <name>
• --rule <name>
• --format json
• --base-url <url>
• --headed
• flexpect init

Generate an example flexpect.yml.