Introduce the foundation for the component-based GraphQL emitter:

- Build/config: Add @alloy-js/core, @alloy-js/graphql dependencies,
  configure JSX transpilation (tsconfig, vitest)
- Context system: GraphQLSchemaContext with ClassifiedTypes,
  ModelVariants, and ScalarVariant interfaces
- Field components: Field, OperationField, and GraphQLTypeExpression
  for rendering model properties and operations as GraphQL SDL
- Type resolution: GraphQLTypeExpression handles scalars, models,
  enums, unions, arrays, and nullability using an isInput prop to
  distinguish input vs output context
- Scalar mappings: Add getGraphQLBuiltinName() for built-in scalar
  identity checks

The ModelVariants lookup structure was originally designed to help
components decide when to append "Input" suffix to model names.
However, the mutation engine now fully handles this:
- Input models are mutated with the "Input" suffix already applied
- Property type references are rewired to point to the correct variants

This commit removes the unused ModelVariants interface and the lookup
logic from GraphQLTypeExpression, simplifying the component to just
use the model's name directly.

Also adds unionMembers to context (needed for union rendering).

Remove ClassifiedTypes, ScalarVariant, unionMembers, and
scalarSpecifications from GraphQLSchemaContextValue. The context now
holds a single TypeGraph — a self-contained namespace of mutated types
that downstream components can walk directly.

Classification (input/output/interface) and scalar metadata are derived
from the TypeGraph at render time rather than pre-computed into parallel
arrays and side-channel maps.