Implement multisig transfer approval workflow with core functionalities and error handling

[BigBen-7]

Multi-Sig Asset Transfer Contract (Requests + Approval Workflow)

Summary

Implements a smart contract for managing high-value asset transfers using transfer requests, configurable multi-signature approval rules, time-based expiry/deadlines, and atomic execution via cross-contract calls to an Asset Registry.

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closes #371

Features

• Transfer requests with lifecycle: Pending → Approved/Rejected/Executed/Cancelled
• Configurable approval rules per asset category:
  • required approvals
  • authorized approvers list
  • approval timeout / deadline
  • rule priority + optional auto-approve
• Approval tracking prevents double approvals
• Reject + cancel flows unlock assets for future requests
• Time-lock support via execute_after
• Asset-level guard prevents multiple pending requests
• Transfer history tracking per asset
• Full event emission for auditability

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Files Added

• contracts/multisig_transfer/src/lib.rs
• contracts/multisig_transfer/src/types.rs
• contracts/multisig_transfer/src/storage.rs
• contracts/multisig_transfer/src/errors.rs
• contracts/multisig_transfer/src/rules.rs
• contracts/multisig_transfer/src/registry.rs
• contracts/multisig_transfer/src/approvals.rs
• contracts/multisig_transfer/src/events.rs
• contracts/multisig_transfer/src/utils.rs
• contracts/multisig_transfer/src/test.rs

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Acceptance Criteria

Transfer requests can be created for existing assets
Approval rules configurable per category
Approvers can approve/reject pending requests
Transfer executes when approval threshold is met
Cross-contract call updates registry ownership
Expired requests cannot be approved/executed
Prevents multiple approvals by same approver
Admin can configure rules
All workflow events emitted correctly
Handles edge cases (expired/cancelled/rejected) properly

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