feat(lock): add distributed lock primitive

[slow2342]

Summary

• Implement distributed lock primitive with lease association and automatic release
• Add AetherLock gRPC service with Lock, Unlock, LockQuery RPCs
• Lock state is persisted in KV store and restored on startup/snapshot

Changes

• proto/lock.proto — Lock service definition with request/response messages
• src/lock/mod.rs — LockManager with reverse indexes for O(1) lookup by key and lease
• src/api/lock.rs — AetherLock gRPC service implementation
• src/raft/mod.rs — LockAcquire/LockRelease Raft request/response types
• src/raft/state_machine.rs — Lock handling in state machine apply path, snapshot restore, and lease revoke

Testing

• Unit tests for LockManager (acquire, release, lease association, snapshot restore)
• Integration tests for lease revoke releasing associated locks
• 118 tests passed, clippy clean

How to verify

cargo build
cargo test --lib
cargo clippy -- -D warnings

Refs #89