Complete the GraphQL capability roadmap

[zth]

Summary

This is stacked on #51 and implements the remaining GraphQL capability roadmap in logical commits.

• modernize the GraphQL runtime contract and native OneOf support
• add input-field and resolver-argument defaults, descriptions, deprecations, and directive metadata
• add schema descriptions, SCHEMA directives, and custom root mappings
• validate emitted SDL with graphql-js and remove invalid union-member description syntax
• add query/mutation/subscription shorthand and non-subscription AsyncIterable list inference
• add custom scalar parseLiteral support
• ship a tested schema-merging compatibility plugin
• ship a config JSON Schema and resgraph check
• expose directive and argument source metadata in hover/definition tooling
• finalize the roadmap with focused designs for generic specialization, derived contexts, and Federation/external schemas

Local verification

• native formatter and Dune build
• bundled CLI build
• full test suite
• runtime directive, OneOf, scalar, shorthand, and compatibility tests
• multi-schema and LSP regressions
• authorization fixtures

CI was intentionally not inspected or awaited.