fix: track blob refcounts so prune does not delete shared files

[softmanmaker]

Summary

• Persist per-hash blob reference counts (blob_refs) so deleting one backup no longer removes blobs still used by other backups (including empty poi/entities .mca files that share MD5 d41d8cd98f00b204e9800998ecf8427e).
• On first startup without the table, backfill counts from existing junctions; if the table already exists, trust it. Empty files are never counted and are never deleted; restore recreates a missing empty blob.
• Add unit tests for backfill, trusted counts, empty MCA prune, rename/shared hash, restore heal, and deleteUnusedBlobs.

Test plan

• ./gradlew :common:test --tests IncrementalPruneReproduceTest
• Upgrade an existing world: confirm blob_refs is created once and later restarts do not rescan
• /xb prune on a world with many empty poi/entities region files; remaining backups still check / restore
• Restore a backup whose empty blob was already missing; target empty files are created

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