feat(shard): add shard management with region splitting

[slow2342]

Summary

Implement region-based data partitioning with metadata tracking through Raft. Regions represent contiguous key ranges [start_key, end_key) with associated leader and replica placement. This lays the foundation for future multi-Raft-group architecture.

Changes

• src/shard/region.rs — Region, RegionEpoch data structures with serde + rkyv dual serialization
• src/shard/manager.rs — ShardManager in-memory index with find_region, apply_split, apply_update, load/save from RocksDB
• src/shard/scheduler.rs — SplitScheduler background task for size-based auto-split (not yet wired)
• src/api/shard.rs — ShardService gRPC: GetRegion, ListRegions, SplitRegion
• proto/shard.proto — AetherShard service definition
• src/raft/mod.rs — RegionSplit/RegionUpdate variants in RaftRequest/RaftResponse
• src/raft/state_machine.rs — ShardManager integration with apply handlers and snapshot support
• src/storage/rocksdb.rs — New region column family
• src/config.rs — ShardConfig (max_region_size_bytes, max_regions, auto_split)
• src/main.rs — Wire ShardService and ShardManager

Design decisions

• Single Raft group: splits are metadata-only operations, no data movement
• Region metadata through Raft: consistent with KV/Lease/Auth mutation pattern
• rkyv + serde: rkyv for storage hot path, serde for Raft bincode path
• max_regions enforcement: checked during apply_split, preserved across snapshot restore

Testing

• cargo build passes
• cargo test passes
• cargo clippy -- -D warnings clean
• cargo fmt -- --check clean

Related issues

Refs #86