Add CITATION.cff and formalize Zenodo release verification

[bernalde]

Context

This is a package-level follow-up to JuliaQUBO/QUBO.jl#66.

• Current GitHub release: v0.7.3
• Existing Zenodo concept DOI: 10.5281/zenodo.7511329
• Current release is archived as DOI 10.5281/zenodo.20926960.
• The README has an evergreen Zenodo badge, but there is no CITATION.cff.

The deposit is current. Remaining work is to formalize citation semantics,
shared management, upstream attribution, and release verification.

Scope

• Verify at least two active JuliaQUBO maintainers have Can manage
  access.
• Add a schema-valid CITATION.cff using the concept DOI as the evergreen
  MQLib.jl software identifier.
• Preserve and document attribution to the upstream MQLib project; avoid
  implying that the wrapper's DOI identifies or archives upstream MQLib.
• Document when users should cite this wrapper, upstream MQLib, and the
  QUBO.jl ecosystem article.
• Document that an exact MQLib.jl release should use its Zenodo version
  DOI.
• Add release-time verification that the corresponding Zenodo archive was
  published under the existing concept DOI.

Acceptance criteria

• Concept DOI 10.5281/zenodo.7511329 remains authoritative.
• At least two maintainers can manage the deposit.
• README and CFF give consistent wrapper/upstream citation instructions.
• The release workflow or checklist detects archival drift.