Implement sharding by key hash across a cluster of nodes, with a coordinator for routing

[AlexJuca]

Context

To scale beyond one node, FKVS needs deterministic routing of keys to nodes. This issue focuses on sharding keys by hash and routing requests through a coordinator or equivalent routing layer.

Related: #28 tracks the broader cluster capability.

Scope

• Choose sharding algorithm: simple modulo, consistent hashing, or another scheme.
• Define coordinator responsibilities and failure behavior.
• Route single-key commands (GET, SET, DEL, TTL/integer commands) to the correct shard.
• Define behavior for resharding and data movement.
• Decide how clients discover coordinator/shard topology.

Acceptance criteria

• Given the same cluster topology, a key maps deterministically to the same node.
• Requests are routed to the owning shard correctly.
• Adding/removing a node has documented data movement behavior.
• Tests cover routing correctness and at least one topology change.