docs(citation): add release DOI badge

[emmepra]

Summary

• add the Zenodo DOI badge to the README header
• update the README citation section to prefer the minted v0.1.0 release DOI
• add a preferred-citation block to CITATION.cff pointing at the current Zenodo DOI

Scope

• in scope:
  • DOI badge wiring
  • DOI-based software citation example
  • GitHub citation metadata for the current archived release
• out of scope:
  • future DOI refreshes for later releases
  • software-paper publication workflow

Validation

• relevant tests/checks were run
• Docker/runtime validation was run when the change touched backend, orchestration, templates, or integration paths
• docs/rules were updated when behavior, contracts, or workflow changed

Commands run:

ruby -e "require 'yaml'; YAML.load_file('CITATION.cff'); puts 'CITATION.cff OK'"

Risks

• user-visible regressions: low; docs/metadata only
• operational/runtime risks: none expected
• follow-up work:
  • repeat the DOI refresh after each new Zenodo-backed release

Evidence

• logs: local CFF parse validation
• screenshots: none
• related issues: none