test: hoist the branch-v1 stamp into a constant (goconst)
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The new dry-run tests added a third "branch-v1" literal, tripping goconst. Name it once as legacyCheckpointVersion.
Co-Authored-By: Claude Opus 4.8 (1M context) noreply@anthropic.com Entire-Checkpoint: 01KWY80X77KHP4HV0Z9EJHFK16
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