Fionabronwen/graphql alloy foundation

[FionaBronwen]

Summary

• Add Alloy JSX build toolchain (@alloy-js/graphql, @alloy-js/cli, rollup plugin, TSX/JSX
  compiler config)
• Introduce GraphQL schema context system (GraphQLSchemaContext, GraphQLTypeResolutionContext)
  for sharing classified types and input/output mode across the component tree
• Add field-level components: Field, OperationField, and GraphQLTypeExpression — the core type
  resolution component that handles scalars, models, enums, unions, arrays, nullability, and
  input/output type splitting
• Fix nullable tracking: track nullability on the container (ModelProperty, Operation) instead
  of the shared type singleton to prevent poisoning all uses of a scalar. Add comprehensive
  module documentation in nullable.ts explaining the architecture.
• Add getGraphQLBuiltinName() for identity-based scalar → GraphQL built-in resolution
• Add operation return type nullable tracking in GraphQLOperationMutation

Details

This is the foundation layer for the Alloy-based GraphQL emitter rewrite. It provides:

Build infrastructure: Alloy CLI for builds, rollup plugin for vitest JSX transform, TSX-aware
TypeScript config.

Context system: Two React-style contexts — GraphQLSchemaContext provides classified types
(models, enums, scalars, unions, operations) and model variant lookups for input/output name
resolution. GraphQLTypeResolutionContext tracks whether we're resolving types in input or
output mode, which determines naming (e.g., Foo vs FooInput) and non-null semantics.

Nullable tracking: The mutation engine strips null variants from unions before components
render. Since the structural evidence of nullability is gone by render time, state maps bridge
the gap. Three tracking sites:

• Property-level: GraphQLModelPropertyMutation marks ModelProperty for inline T | null
• Operation-level: GraphQLOperationMutation marks Operation for op foo(): T | null
• Type-level: GraphQLUnionMutation marks the new union object for union Pet { Cat, Dog, null }

See nullable.ts module doc for the full architectural explanation.

Field components: Field renders model properties as gql.Field/gql.InputField. OperationField
renders operations as fields with arguments. Both delegate type resolution to
GraphQLTypeExpression, which uses typekit predicates for type narrowing and render-prop
children for structured output.