feat: multisig proposal expiry

[anonfedora]

Multisig Proposal Expiry

Overview

The contract includes a minimal multisig admin module used for sensitive administrative changes (e.g. freezing the contract, rotating owners, changing threshold, changing proposal duration).

Each multisig proposal has a deterministic expiry timestamp stored on-chain. After expiry, the proposal can no longer be approved or executed.

Storage and Types

• Proposals are stored in persistent storage under DataKey::MultisigProposal(u32).
• Proposal payload is represented by:
  • ProposalAction (the requested admin change)
  • Proposal (proposal metadata including approvals and expiry)

The Proposal struct includes:

• id: u32
• action: ProposalAction
• proposer: Address
• approvals: Vec<Address>
• executed: bool
• expiry: u64

Expiry Semantics

How expiry is computed

When creating a proposal via propose_action, expiry is calculated as:

• now = env.ledger().timestamp()
• expiry = now + proposal_duration

Closes #150

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