Feature/contracts 35 escrow closure finalization

[gloskull]

PR: feat/contracts-35-escrow-closure-finalization

Summary

Implements Escrow contract closure finalization with immutable close records and summary metadata.

What changed

• contracts/escrow/src/lib.rs

  • EscrowContract now includes:
    • finalized_at: Option<u64>
    • finalized_by: Option<Address>
    • close_summary: Option<Symbol>
  • Added finalize_contract method with:
    • status precondition (Completed | Disputed)
    • one-time finalization guard (immutable once performed)
    • participant authorization guard (client/freelancer/arbiter)
  • Added read helpers:
    • is_finalized
    • get_close_summary
    • get_finalizer

• contracts/escrow/src/test.rs

  • Added tests:
    • test_finalize_contract_success_and_immutable
    • test_finalize_contract_already_finalized
    • test_finalize_contract_not_ready
    • test_finalize_contract_unauthorized

• README.md and docs/escrow/status-transition-guardrails.md

  • Documented finalization workflow and guardrails

Security notes

• Finalization allowed only after final applicant status; prevents premature closure.
• Finalization is immutable after the first call.
• Caller must be a known contract participant.

Proof of successful build/tests

Closes #85

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[mikewheeleer]

@gloskull please resolve the conflicts

[gloskull]

@mikewheeleer please merge

[gloskull]

@mikewheeleer please merge

[gloskull]

@mikewheeleer please merge