Generalize indexing schema versions

[jwils]

Why

The indexing event decoder contract should not force non-JSON transports to expose JSON-specific schema versioning. Proto/Kafka decoders can omit schema_version, while JSON ingestion owns the json_schema_version mapping when needed.

What

• Replace indexer event/output usage of json_schema_version with generic schema_version
• Add a JSON ingestion JSON Lines decoder that maps json_schema_version to schema_version
• Add schema artifact wrappers for generic version selection and envelope validation adaptation
• Update warehouse partition/log naming, docs, RBS, and tests for the generic event contract

Risk Assessment

Medium: this changes core indexing event normalization and warehouse log keys, but JSON ingestion compatibility is preserved through the new decoder and the affected unit suites pass.

References

• Stacked on Add configurable indexing event decoder #1220
• Verification: affected unit suites, script/type_check, and script/lint