Rack names are only unique within a data center — two DCs can both hold a
"rack1" — so ShardsByRack, RackToShards, and RackToNodes keyed by the bare
rack name merged distinct domains and corrupted both the DC-level shard
counts and rack-level balancing. Rack lookups now use the dc/rack pair;
DCToRacks still lists bare rack names.
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Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>