feat(setup): clarify ROMarr setup paths

[snapetech]

Summary

Make ROMarr’s existing setup surfaces clearer about when an indexer is needed to search, when a download client is needed to grab, and how the client-reported completed path maps to the library root inside the container. Keep browsing/local-library work usable when acquisition services are absent. Add optional follow-ups for DAT checks, import lists, plugins, and playback integrations in the existing Get Started page and install guide. The per-repo contract records ROMarr’s specific first-use path.

Also sync the existing runner-policy guard test with the repository’s current exact runner assignments: the public-fork JIT runner JSON and the dispatch-only, main-only gVisor smoke job.

Merge

Target main; merge after repository-required checks pass. The maintainer explicitly directed the commit, push, and merge.

Validation

• Python AST parse and locale-label JSON checks passed.
• python3 -m pytest tests/test_workflow_runner_policy.py -q: 6 passed.
• git diff --check passed.
• The full repository suite is running through this PR’s required checks.

Release note

• I added a validated release-note fragment under release-notes/ for this user-facing change.
• This change is internal-only and needs no release-note fragment.