Add `set_treasury` two-step rotation

[greatest0fallt1me]

Description

This is a smart-contract issue for the GrantFox campaign. This is a smart-contract issue for the GrantFox campaign. Two-step treasury rotation for revenue pool.

Requirements and Context

• Implement per the description
• Add focused tests for the change
• Document any API/visible changes
• Adhere to repo's lint and code style
• Must be secure, tested, and documented
• Should be efficient and easy to review

Suggested Execution

1. Fork the repo and create a branch

   git checkout -b feature/set-treasury

2. Implement changes
   • contracts/revenue_pool/src/admin.rs
3. Test and commit
   • Run the repo's standard test suite and lint
   • Cover edge cases; include output in the PR

Example commit message

feat: two-step set_treasury

Acceptance Criteria

• Implementation matches the description
• Tests added and passing
• Code review approved
• Docs updated

Guidelines

• Minimum 95% test coverage with cargo test
• require_auth on every state-changing entrypoint
• Overflow-safe math; no unwrap() in production paths
• Clear NatSpec-style /// rustdoc
• Timeframe: 96 hours