sqllogictest: rewrite relations.slt golden for Arc-backed batches

The introspection golden prints the Rust type name of each arrangement's batch
allocation. Switching the production spines to `ArcBatch` changes that name from
`alloc::rc::Rc<OrdValBatch<..>>` to
`mz_row_spine::arc_batch::ArcBatch<OrdValBatch<..>>`. Churn only, 42 symmetric
type-name substitutions, no operator-graph change.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019G29DBfgE8LXpE5jamm2Zi

Author: antiguru