[NF-07] Add Guardian-gated Emergency Pause / Circuit Breaker to VaultRouter

[Leothosine]

Summary

Adds a Guardian-gated circuit breaker to VaultRouter so all mutating operations can be halted instantly during an active incident, without waiting for the 72-hour governance timelock.

Changes

contracts/vault_router/src/error.rs

• Added ProtocolPaused = 4 and Unauthorized = 5 error variants

contracts/vault_router/src/lib.rs

• Added DataKey::Guardian and DataKey::Paused (bool, default false) storage keys
• initialize now accepts a guardian address and sets Paused = false
• New pause() - Guardian-only; sets Paused = true and emits a paused event
• New unpause() - Guardian-only; sets Paused = false and emits an unpaused event
• New is_paused() - read-only view, not gated
• Added require_not_paused() internal helper called at top of deposit, withdraw, and early_exit
• position() and other read-only functions deliberately not gated (users can still see funds while paused)
• Full test suite updated: setup() passes guardian, 7 new tests covering all acceptance criteria (deposit/withdraw/early_exit rejected while paused, deposit succeeds after unpause, position readable while paused, non-guardian pause rejected)

closes #82

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