Add Treasury Rotation Event and Tests

[Naomi-Gift]

Treasury can be set, but treasury changes are not treated as a first-class audited operation.
Acceptance criteria:
Emit a treasury updated event.
Include old and new treasury where possible.
Add tests for admin-only treasury updates.
Document treasury rotation behavior.

[clintjeff2]

Hello, I have 6 years of experience working on tasks like this and handling similar challenges across backend, frontend, and smart-contract development. I have carefully read and fully understood the task requirements, and I am confident I can deliver a clean, efficient, and well-tested solution promptly. I always ensure high-quality implementation, maintainability, and adherence to best practices.

Pull request in 5 hours.