fix: merge interface fields into derived types in SDL generation

[kleberbaum]

Summary

Fixes #110 — inherited/implemented fields are not included in the SDL for derived types, causing GraphQL schema validation to fail at runtime.

Problem

When class Traffic extends RegionNode, Pylon correctly:

1. Detects the inheritance relationship
2. Creates the IRegionNode interface with all fields
3. Adds implements IRegionNode to Traffic

But it does not copy the interface fields into Traffic's field list in the SDL. GraphQL-js validateSchema then fails:

Interface field IRegionNode.id expected but Traffic does not provide it.

Fix

In addInterfaceToDerived (schema-parser.ts), after adding the implements clause, merge all interface fields into the derived type's field list (skipping fields already declared on the derived type).

if (targetInterface) {
  for (const ifaceField of targetInterface.fields) {
    const alreadyHasField = derivedSchemaType.fields.some(
      f => f.name === ifaceField.name
    )
    if (!alreadyHasField) {
      derivedSchemaType.fields.push({...ifaceField})
    }
  }
}

Test plan

• All 22 existing schema tests pass (inheritance, generics, basic-features, inputs-and-args)
• Verified at runtime on Cloudflare Workers with a real-world GTFS GraphQL API using Traffic extends RegionNode and Tourism extends RegionNode

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