feat: implement pausable functionality for critical contract operations

[BigJohn-dev]

Closes #25

Summary

Completes pausable functionality for critical contract operations in the renaissance-vault contract, bringing it in line with renaissance-betting and renaissance-rewards (which already ship a Pausable utility).

The vault already had the infrastructure — a Paused storage key and an ensure_not_paused guard on its critical functions — but the wiring was incomplete: there were no public pause()/unpause() entry points, so the vault could never actually be paused, and pause failures surfaced as a misleading InvalidAmount error.

Changes

vault/src/lib.rs

• Public pause() / unpause() / is_paused() functions, gated to the vault admin via a new require_admin helper (Address::require_auth). pause() is idempotent.
• pause/unpause emit Paused / Unpaused events logging the admin account that triggered the action.
• Added a clear VaultError::Paused (208) variant (plus Unauthorized (209)). ensure_not_paused now returns Paused directly instead of masquerading as InvalidAmount.
• The four critical state-changing functions — deposit, withdraw, lock_for_bet, payout — already called ensure_not_paused, so they now revert with the clear Paused error when the vault is paused.

vault/src/test.rs

• The pre-existing test file no longer compiled against the pinned soroban-sdk 21.4.0 (stale .get_address() / .mint() / AddressEnvTestUtils API, 87 errors). Rewrote it to the current SDK API, matching the working patterns in the betting crate, preserving all prior coverage.
• Added Pausable unit tests:
  • pause/unpause toggle state; pause idempotency
  • Paused / Unpaused events emitted and logging the admin
  • pause requires admin auth
  • deposit / withdraw / lock_for_bet / payout revert with Paused while paused
  • operations resume after unpause

Acceptance criteria

• Contract includes a standard Pausable utility
• pause() / unpause() protected by an admin/owner check
• Critical state-changing functions guarded by the not-paused check (deposit, withdraw, lock_for_bet, payout)
• Calling a pausable function while paused reverts with a clear error (VaultError::Paused)
• pause() / unpause() emit Paused / Unpaused events logging the triggering account
• Unit tests verify correct execution when not paused and reverts when paused

[Divineifed1]

@BigJohn-dev fix CI