Implement stream collateral mode where the deposit is locked and earns yield via a DeFi vault integration

[Chuks-coderr]

Description

Add an optional collateral mode at stream creation that routes the deposited funds into an approved on-chain yield vault. The accrued yield is split between sender and recipient according to a configurable ratio, while the principal remains available to back the stream's payment obligations.

Acceptance Criteria

• Feature is fully implemented
• Tests pass
• Lint/type checks pass
• PR references this issue number

Notes

⚠️ Do not begin coding until you have been assigned by the maintainer.
Branch as feat/472-contract-collateral-yield where N is this issue number.