Clarify Warpkeep Assets licensing

[ael-dev3]

Summary

• add a root mixed-license map for Warpkeep-Assets
• apply the main Warpkeep repository's Apache-2.0 license to project-authored verification tooling
• make the two existing CC BY 4.0 asset grants easy to find
• explain archive-only, external, unresolved, and trademark boundaries in plain language
• link the main Warpkeep licensing policy and runtime asset inventory
• add regression coverage for the repository license map and bundled license texts

Scope

This PR does not relicense asset payloads, release attachments, manifests, provenance records, archive-only deposits, third-party material, or Warpkeep trademarks. Existing per-set terms remain authoritative.

Validation

• python3 -m compileall -q -f scripts tests
• python3 -m unittest discover -s tests -v — 34 tests passed
• git diff --check
• bundled Apache-2.0 text matches the main Warpkeep repository