feat(primitive): add barrier and queue primitives

[slow2342]

Summary

• Add distributed barrier primitive with blocking semantics (create/release/query)
• Add distributed FIFO queue primitive (enqueue/dequeue/peek)
• Both primitives integrate with Raft state machine, lease system, and watch mechanism

Changes

• proto/barrier.proto — Barrier gRPC service definition (Create, Release, Query)
• proto/queue.proto — Queue gRPC service definition (Enqueue, Dequeue, Peek)
• src/barrier/mod.rs — BarrierManager with 4-map index pattern, restore, lease integration
• src/queue/mod.rs — QueueManager with sequential key generation, restore from meta prefix
• src/api/barrier.rs — BarrierService with create_with_wait blocking semantics
• src/api/queue.rs — QueueService implementing AetherQueue tonic trait
• src/raft/mod.rs — Added RaftRequest/RaftResponse variants for barrier and queue
• src/raft/state_machine.rs — Apply methods, dispatch arms, lease cleanup, watch events
• src/main.rs — Manager instantiation and gRPC service registration
• build.rs — Added barrier.proto and queue.proto to compilation
• src/lib.rs, src/api/mod.rs — Module declarations and re-exports

Design

Barrier: Similar to lock — create holds a barrier that blocks other callers. Supports lease-based auto-release. Uses watch-based create_with_wait for blocking semantics (like election's campaign_with_wait).

Queue: Sequential keys (_aether_queue/<name>/<seq>) with meta key tracking next sequence. Dequeue scans prefix for front item and deletes atomically. Peek reads without removing.

Testing

• All 146 unit tests pass
• cargo clippy -- -D warnings passes
• cargo fmt -- --check passes

Related issues

Refs #91