Add regression test for the #781 dispute-arbitration hardening fix

[fejilaup-cloud]

Labels: testing, atomic-swap, regression
Priority: Medium
Estimated Time: 1.5 hours

Description:
Recent commits (63daf53, PR #799) hardened dispute arbitration and added missing ContractError variants. regression_tests.rs (329 lines) should capture this fix permanently so it can't silently regress in a future refactor.

Tasks:

• Add a named regression test referencing Dispute resolution is a single-key instant ruling with none of the threat model's documented safeguards implemented #781 in regression_tests.rs
• Assert every ContractError variant referenced by arbitration_tests.rs actually exists and round-trips through Error::from_contract_error
• Document the fix in docs/threat-model.md's changelog section

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[Peculiarr7]

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