Add unit tests for Storage and ClusterManager

[meonthewire]

Summary

Core storage engine has no test coverage

src/storage/src/storage.rs has zero tests. This is the central data layer of the project key-value operations, TTL and expiration, batch operations, counter arithmetic, list and set data structures, CRDT logic, eviction policies, WAL recovery, and compaction are all untested.

Cluster manager has no test coverage

src/cluster/src/manager.rs is the largest file in the codebase and also has no tests. Replication entry application, pending replication queue behavior, peer heartbeat and health transitions, and serialization of replication data are all unverified.

Acceptance Criteria

• cargo test --all passes with new tests
• Each data type operation has at least 2 test cases (happy path + edge case)
• WAL recovery test verifies data survives simulated crash