Fix bootstrap disk and network amplification when replicating artifacts across peers

Kura now deduplicates artifact applies during bootstrap and live replication. When multiple peers concurrently serve the same artifact, a per-key write lock ensures the body is only written to disk once, preventing the store from growing to many times the dataset size and running out of disk space. A per-artifact fetch gate also single-flights the download across peers, so a fresh node transfers roughly 1× the dataset over the network instead of N×. This fixes the ENOSPC / “insufficient free space for segment rotation” errors that could block new replicas from reaching Ready.