Implement multi_sig contract - multi-signature sweep authorization

[phertyameen]

Description:

Currently, sweep authorization in sweep_controller relies on a single Ed25519 key. This creates a single point of failure - if the authorized signer is compromised, all sweep operations are at risk. A multi-signature contract requiring M-of-N approvals significantly raises the security bar for high-value disbursements.

Why it's needed:

Single-signer authorization is insufficient for production financial infrastructure. M-of-N signing is a standard safety requirement in custodial and semi-custodial payment systems.

File locations:

contracts/sandbox/multi_sig/Cargo.toml
contracts/sandbox/multi_sig/src/lib.rs
contracts/sandbox/multi_sig/src/storage.rs
contracts/sandbox/multi_sig/src/errors.rs
contracts/sandbox/multi_sig/src/events.rs

Acceptance Criteria:

• Contract initializes with a list of authorized signers and a threshold M
• submit_approval records a signer's approval for a given sweep ID
• Duplicate approvals from the same signer are rejected
• Threshold reached event is emitted when M approvals are collected
• is_approved query returns correct state for a given sweep ID
• Unit tests cover threshold reached, duplicate rejection, and unauthorized signer cases