security(contract): harden upgrade verification and rollback guarantees

[arisu6804]

Problem

An unauthorized or incompatible upgrade can corrupt escrow balances and transfer state.

Objective

Deliver a production-quality improvement to upgrade authorization and artifact verification that creates measurable value for correctness, security, reliability, performance, or maintainability.

Implementation scope

• Require governed authorization, verify approved artifact identity, emit metadata, and guarantee verification failure before state mutation.

Acceptance criteria

• Unauthorized or mismatched upgrades fail atomically; successful upgrades are auditable; rollback behavior is tested.

Required validation

• Authorization, hash mismatch, state preservation, event, and rollback tests.
• Existing tests and CI remain passing.
• Add regression coverage for the original failure mode.
• Do not weaken, delete, or skip unrelated tests to obtain a green build.

PR quality bar

• Keep the PR focused and explain design tradeoffs, compatibility impact, and test evidence.
• Avoid typo-only, documentation-only, cosmetic-only, or unrelated refactor submissions.

Out of scope

• Broad rewrites not required by the acceptance criteria.
• Changes to unrelated services, contracts, or user flows.