Drift item #6: verify HashMap::clear()/remove()/replace drop CachedKey values
triggering ZeroizeOnDrop. Adds drop_tracker module proving Drop semantics,
plus LRU eviction, TTL expiry, and clear() tests. The lock()-clears-cache
criterion is covered by existing test_lock_clears_all_cache_entries in service.rs.
Refs: docs/architecture/crates/vault/README.md drift #6
Replace rand::random() with rand::rngs::OsRng for cryptographic nonce
and salt generation in encryption.rs. rand::random() uses thread-local
RNG which may not be a CSPRNG on all platforms; OsRng reads from the
OS entropy source, preventing catastrophic IV reuse under AES-GCM.
Drift item #1 (security-critical).
Rename the crate from alknet-secret to alknet-vault to better reflect its
purpose as a local key vault (seed management, key derivation, encryption)
rather than a network service.
Symbol renames:
- SecretService → VaultService
- SecretServiceHandle → VaultServiceHandle
- SecretServiceActor → VaultServiceActor
- SecretServiceError → VaultServiceError
- SecretProtocol → VaultProtocol
- SecretMessage → VaultMessage
- ServiceLocked → VaultLocked
- alknet_secret → alknet_vault (crate name)
Update ADR-008 with vault access pattern: the vault is a capability source,
not a service endpoint. The CLI injects derived/decrypted material into
operation contexts — handlers never hold vault references.
