upgrade-governance: add upgrade rollback proposal type

[Emeka000]

Summary

Once an upgrade is executed there is no on-chain mechanism to propose and vote on reverting to a previous WASM hash. If a bug is discovered post-upgrade, rollback requires manually re-proposing the old hash without any rollback semantics or guards.

Proposed Change

• Add propose_rollback(signer, target_proposal_id) that creates a rollback proposal pre-populated with the WASM hash and metadata from a previously executed proposal
• Rollback proposals require the same signer threshold as standard upgrades
• Emit a UpgradeRollbackProposed { rollback_proposal_id, original_proposal_id, wasm_hash } event
• Add max_rollback_age_ledgers config — rollback to proposals older than this is blocked

Acceptance Criteria

• Rollback proposal references original proposal ID
• Rollback blocked for proposals older than max age
• Rollback follows same voting and timelock flow as regular upgrade
• Test covers: propose, vote, execute rollback cycle