Add schema universe - #4422

Summary

This PR introduces an experimental Schema Universe for storing many related GraphQL schemas with structural sharing. It also adds a schema-neutral ExecutableSchema / Schema* view so common schema consumers can operate on either a GraphQLSchema or a schema-universe snapshot without duplicating their core logic.

The implementation is aimed at workloads that derive hundreds or thousands of schemas from a common source through small transformations.

Schema Universe

• Adds SchemaUniverse, an append-only chunked vertex arena with stable integer vertex IDs.
• Represents relationships as packed primitive edges stored outside vertices.
• Publishes immutable SUSchema snapshots backed by persistent integer maps, sharing unchanged adjacency, named-type indexes, implementation indexes, and metadata.
• Supports importing from GraphQLSchema, SDL, and TypeDefinitionRegistry, as well as exporting back to GraphQLSchema.
• Supports schema derivation through SUSchemaBuilder, including ordered applied directives and applied directives on directive definitions.
• Stores all named types directly in each snapshot and maintains an interface-to-implementations index for efficient possible-type lookups.
• Includes per-schema introspection types, including customized introspection definitions.
• Retains schema and element AST definitions in optional chunk-aligned sidecars.
• Adds sparse, user-controlled metadata keyed by vertex ID on each schema snapshot.
• Defines schema registry lifetime semantics, schema removal, and cleanupUnusedVertices() for reclaiming vertices unused by registered schemas while preserving stable IDs.
• Adds design documentation and JMH/JOL benchmarks for import, transformation, lookup, and retained-memory analysis.

Executable schema views

• Adds the experimental ExecutableSchema API and a schema-neutral Schema* type hierarchy.
• Makes GraphQLSchema implement the new interfaces directly.
• Adds SUExecutableSchema, an adapter over SUSchema that provides schema-specific visibility, scalar coercion, enum coercion, introspection, and possible-type behavior.
• Generalizes SchemaPrinter to print complete executable schemas and individual schema elements from either representation while preserving the existing GraphQL-specific filtering and comparator APIs.
• Adds schema-neutral filtering and comparator options with defaults matching the existing GraphQL ordering.
• Generalizes parsing/validation, operation validation, field collection, conditional directives, AST type resolution, and input value coercion to use ExecutableSchema and Schema* elements.
• Keeps the existing GraphQL-specific public overloads and behavior intact; they delegate to the shared implementation where appropriate.

Scope

This remains experimental and does not yet replace GraphQLSchema in request execution. A schema-universe view can currently be printed, parsed/validated against, traversed for field collection, and used for variable/argument/input coercion. Wiring GraphQL and execution strategies to execute against SUExecutableSchema is intentionally left for follow-up work.

The current universe import/export also does not preserve execution-only GraphQLSchema state such as the code registry.

Testing

• Adds focused tests for packed edges, persistent maps, schema construction/transformation, cleanup, metadata, ordered directives, AST retention, implementations, possible types, introspection, and round trips.
• Runs schema printing, validation, field collection, conditional-node handling, and value coercion against both GraphQLSchema and SUExecutableSchema paths.
• The full local test suite passes (6,093 tests).
• CI passes on Java 11, 17, 21, and 25, including the per-class coverage guard and jcstress.