Polish JSON ingestion extension APIs and docs

[jwils]

Why

Apply final JSON ingestion polish after the extraction and make the docs match the explicit extension-module setup.

What

• Wrap JSON ingestion indexing field types instead of globally prepending behavior onto core field type classes
• Add the JSON-ingestion union type extension and related RBS signatures
• Document JSON ingestion schema-definition APIs and update repository guidance
• Update Apollo/Warehouse examples to append their extension modules without removing JSON ingestion
• Ensure JSON-specific test helpers request the JSON ingestion extension explicitly

Risk Assessment

Medium — this is mostly documentation and internal extension-boundary cleanup, with focused coverage for the JSON schema behavior.

References

• Stacked on Use JSON ingestion as schema definition extension #1205.
• Local lint, type check, spellcheck, targeted specs, schema artifact dump/check, and documentation coverage passed.
• Full local script/quick_build could not run because Docker and docker-compose are not installed, so no local datastore can be booted; GitHub CI should cover that environment.