Add @stdlib/json_schema module for JSON Schema validation

[nbeerbower]

Summary

Adds a new @stdlib/json_schema module to the standard library, bringing JSON Schema validation capabilities to Hemlock. This is particularly useful for validating structured output from LLMs, API responses, configuration files, and user input.

Key Changes

• New module: stdlib/json_schema.hml - Pure Hemlock implementation of JSON Schema Draft 7 validation (400+ lines)
• Comprehensive documentation: stdlib/docs/json_schema.md with API reference, examples, and use cases
• Schema builders: Helper functions for constructing schemas programmatically (string_type, object_type, array_type, nullable, enum_type, etc.)
• Core validation functions:
  • validate(value, schema) - Returns detailed error information with paths
  • is_valid(value, schema) - Boolean-only check
  • validate_json(json_str, schema) - Parse and validate in one call
  • format_errors(errors) - Human-readable error formatting
• Supported schema keywords: type, enum, const, required, properties, items, additionalProperties, minimum/maximum, minLength/maxLength, minItems/maxItems, uniqueItems, allOf, anyOf, oneOf, not, if/then/else
• Comprehensive test suite: 8 test files covering arrays, objects, numbers, strings, composition, conditionals, and error reporting
• Updated documentation: CLAUDE.md and stdlib/README.md reflect the new 43rd module

Implementation Details

• Built as a pure Hemlock module on top of @stdlib/json with no external dependencies
• Deep equality comparison for enum/const validation
• Detailed error paths using dot notation (e.g., users[1].name)
• Type checking that handles Hemlock's numeric type variants
• Composition keywords (allOf/anyOf/oneOf/not) with proper short-circuit evaluation
• Conditional schemas (if/then/else) for advanced validation logic

Use Cases

• LLM structured output validation - Ensure LLM responses match expected JSON shapes
• API response validation - Verify third-party API responses conform to contracts
• Configuration validation - Validate config files at load time
• Tool call argument validation - Validate arguments passed to tool functions

https://claude.ai/code/session_01Vrk4CoN6sAZBCRxvzyuBpY