[NF-08] Add Emergency Unlock mode to vault_l3

[bellabuks]

Summary

Adds a Guardian-gated Emergency Unlock mode to vault_l3 so depositors can exit locked positions fee-free during a declared incident, without waiting for maturity.

Changes

• contracts/vault_l3/src/lib.rs: added DataKey::Guardian and DataKey::EmergencyUnlock (bool, default false)
• initialize now accepts a guardian address
• New set_emergency_unlock(active: bool) - Guardian-only toggle, independent of the NF-07 pause flag
• New emergency_unlock() -> bool read-only view
• early_exit: when EmergencyUnlock is active, skips the 0.5% fee and returns full principal
• withdraw: when EmergencyUnlock is active, skips the LockNotExpired check
• Balance and share accounting is unchanged in both paths
• contracts/vault_l3/src/test.rs: updated setup() + 6 new tests covering all acceptance criteria

closes #83

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[wumibals]

very good