[beinan]

Summary

Ingress and materializers need the same schema identity after metadata is reloaded in a different map order. Add an optional arrow-contract helper that canonicalizes Arrow 58 schema JSON before applying the existing ingestion-contract hash. It sorts nested object keys while preserving field order, types, nullability, metadata values and dictionary attributes.

The helper needs schema metadata only. Replicas can keep using the Arrow-free contract feature. CI covers both JSON map-order configurations; fixed canonical bytes and an independently checked digest detect representation changes during dependency upgrades.

Refs #378.

Testing

• Four new schema tests passed, including fixed bytes/hash, nested metadata/reload stability, meaningful schema changes and dictionary attributes.
• Optional-feature release tests passed with both JSON map-order configurations; release Clippy passed.
• Full repository check script passed: formatting, workspace Clippy and all-target Rust tests (594 passed); Python tests (229 passed, 2 skipped, 1 xfailed), Ruff formatting/lint and Pyright.