Add CEL function to check if component is a direct dependency of project root

## Current Behavior
There is CEL support for component-to-component direct edge checks using component.is_direct_dependency_of(v1.Component{...}), but there is no CEL function to directly determine whether the currently evaluated component is a direct dependency of the project root.

## Proposed Behavior
Expose a CEL helper for project-root direct dependency checks, for example:
- component.is_project_direct_dependency()
or
- project.has_direct_dependency(component)

## Why
This enables policy authors to target first-level dependencies only, without requiring explicit parent component matching or brittle workarounds.

## Notes
- Existing helper project.depends_on(...) is not a direct-edge-only check.
- Existing helper component.is_direct_dependency_of(...) works when a specific parent component is known.