fix: correct ArtifactRef import path and UI module dependencies

[m2papierz]

Summary

Fixes two import bugs that cause ModuleNotFoundError:

1. ArtifactRef lazy import pointed to non-existent devqubit_engine.core.models instead of devqubit_engine.core.types
2. devqubit-ui imported from devqubit metapackage instead of devqubit_engine directly, breaking standalone UI installation

Type of change

• Bug fix
• New feature
• Refactor (no user-facing change)
• Docs only
• CI/build tooling
• Breaking change

Changelog (user-facing only)

• I added a towncrier fragment in changelog.d/

Checklist

• I ran lint locally (uv run pre-commit run --all-files)
• I ran tests locally (uv run pytest)
• I added/updated tests for behavior changes
• I updated docs/README/examples if needed
• If I changed dependencies, I updated uv.lock (uv lock) and committed it
• I considered backward compatibility and security impact

Notes for reviewers

No breaking changes - these are pure bug fixes. The UI now imports directly from devqubit_engine submodules, which avoids potential circular dependency issues.