[beinan]

Summary

Ingress and materializers need the same immutable routing keys, schema identity, write policy and record limits before accepting a table generation. Add an independent contract feature with serializable metadata, explicit validation errors and a domain-separated schema fingerprint. It requires neither Arrow nor a replication engine. Callers still enforce durable binding, validate actual payloads and provide authentication and ID uniqueness.

Fixed synthetic binary/hash fixtures preserve the metadata representation. CI exercises the optional feature. Production ingestion is unchanged.

Refs #378.

Testing

• Contract tests cover a fixed bincode representation, unknown policy rejection, independently checked schema hashing, UTF-8 byte bounds and invalid decoded limits.
• Optional-feature tests and release Clippy passed.
• Full repository check script passed: workspace formatting, Clippy and all-target Rust tests; Python pytest, Ruff formatting/lint and Pyright.