feat(contracts): add governed upgrade manager for deployed contracts

[petahade]

Summary

Adds a new upgrade-manager Soroban contract that implements a governed, proxy-equivalent upgrade lifecycle for deployed ArenaX contracts. Soroban has no delegatecall-style proxy — a contract's address stays constant while its wasm implementation can be swapped via Env::deployer().update_current_contract_wasm(hash). This contract is the governance layer in front of that primitive.

• Proxy pattern implementation — stable contract address across wasm swaps via update_current_contract_wasm; other contracts integrate by reading get_approved_wasm_hash and applying it to themselves in their own admin-gated upgrade() entrypoint.
• Upgrade governance vote — propose_upgrade → vote_upgrade (one vote per governor) → execution requires approval_threshold yes-votes.
• Implementation validation — validate_upgrade records a validation verdict; execution is blocked until a positive verdict is recorded.
• Rollback capability — rollback_upgrade restores the previously-approved wasm hash for a contract (or re-applies it immediately for the manager's own implementation), with a recorded reason.
• Upgrade history logged — every execution/rollback appends to a per-contract get_upgrade_history log, and each lifecycle step emits a versioned event via a new arenax-events::upgrade_manager module.

Also registers the new event namespace in arenax-events' events_registry list (lib.rs, events_registry.rs) alongside the existing domains, and adds upgrade-manager to the contracts workspace members.

See contracts/upgrade-manager/README.md for the full design rationale and an integration example for a target contract's own upgrade() entrypoint.

Test plan

• cargo build / cargo test across the contracts workspace (not run in this environment — no Rust toolchain available here)
• Exercise the propose → validate → vote → execute flow against testnet
• Exercise rollback after a bad upgrade

Closes #969

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