Function register 

pub fn register(
    store: &UnifiedStore,
    event_name: &str,
    schema_json: &str,
) -> Result<u32, SchemaError>

Register a schema for event_name.

• First registration → returns version 1.
• Additive successor → returns previous_version + 1.
• Anything else → SchemaError::BreakingChange { offenders } with every break the diff turned up so the caller can fix them all in one round-trip.