Add JSON Schema extraction via schema() builtin function

[nbeerbower]

Summary

This PR adds support for extracting JSON Schema-compatible objects from Hemlock's define types via a new schema() builtin function. This enables validation of JSON data against Hemlock's structural types.

Key Changes

• New builtin function schema(type_name: string) - Extracts type metadata from the runtime type registry and returns a JSON Schema-compatible object with type, name, properties, and required fields

• Compiler support - Added codegen for the schema() builtin in the compiler backend with proper type checking (returns CHECKED_OBJECT)

• Interpreter support - Implemented builtin_schema() in a new schema.c file with type mapping functions for both JSON Schema types and Hemlock type names

• Runtime support - Added hml_schema() function in the runtime's builtins_types.c for the compiled code path

• Standard library helpers - Added three convenience functions to @stdlib/json:

  • to_json_schema(type_name) - Wrapper around the builtin
  • validate_schema(data, schema_obj) - Validates data against a schema object
  • validate_type(data, type_name) - Convenience combining both above functions

• Documentation - Updated stdlib/docs/json.md with comprehensive schema validation guide including usage examples and validation rules

• Tests - Added comprehensive test coverage:

  • tests/parity/builtins/schema.hml - Tests basic schema extraction for multiple define types
  • tests/stdlib_json/schema_validation_test.hml - Tests validation functions with valid/invalid data, missing fields, type mismatches, etc.

Implementation Details

• Schema objects include both JSON Schema types ("string", "integer", "number", etc.) and Hemlock type names ("i32", "f64", etc.) for better type information
• Optional fields are marked with required: false in the schema
• Custom object types include a $ref field pointing to the referenced type name
• Validation supports duck typing (extra fields accepted) and type category matching (all integer types match "integer" schema type)
• Both compiler and interpreter backends are fully supported

https://claude.ai/code/session_012dzceQ2ppdmkSym3fxLAWA