Emit neutral JSON schema artifact (db/event_schema.json)

[tylercschneider]

Summary

Adds a parallel, neutral JSON artifact — db/event_schema.json — generated from the same compiled schema as db/event_schema.rb, so BI/data consumers can read every event's shape and version without running Ruby or evaluating the executable schema file.

Purely additive: the Ruby artifact format, the boot/runtime path, the DSL, and the helper signatures are all unchanged. The JSON is written during the same schema dump.

What changed

• Schema#to_h — plain-data hash parallel to to_ruby: event_name, event_version, event_type, process_type, subject, domain, required_inputs, optional_inputs, payload_fields (each name/from/attr/required), and fingerprint.
• EventSchemaJsonWriter — serializes a full compiled EventSchema (all events, all versions) to JSON, one object per event+version, sorted [event_name, event_version] like EventSchemaWriter. Output is byte-identical on unchanged re-dump.
• EventSchemaDumper.dump! takes an optional json_path: and writes the JSON when given.
• event_engine:schema:dump rake task writes db/event_schema.json next to db/event_schema.rb.
• README documents the JSON contract.

Testing

• New unit tests for to_h, the JSON writer (ordering + determinism), and the dumper wiring.
• bundle exec rake test — 162 runs, 0 failures.

Out of scope (per the issue)

Making JSON canonical at runtime, drift-guard coverage of the JSON file, RBS generation, and conforming to a formal JSON Schema meta-schema — each a separate task.

Closes #157. Part of the #152 plan.

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