feat(swift): Add Swift SDK implementation

[clemensv]

Summary

Add a Swift SDK for JSON Structure schema and instance validation.

Background

Swift is the primary language for Apple's ecosystem (iOS, macOS, watchOS, tvOS, visionOS) with 1.5B+ active devices. Mobile apps frequently need to validate API responses against schemas, making this a high-value addition to the SDK family.

Implementation Guide

Follow the SDK Guidelines document: SDK-GUIDELINES.md

This document contains:

• Required components (SchemaValidator, InstanceValidator, ErrorCodes, JsonSourceLocator)
• Complete type system reference (34 primitive types + compound types)
• Keywords reference (what to implement, what NOT to implement)
• Error codes reference (assets/error-messages.json)
• Test assets for conformance testing
• Conformance checklist

Requirements

Core Features

• Schema Validation: Validate JSON Structure schema documents for conformance
• Instance Validation: Validate JSON instances against JSON Structure schemas
• Error Reporting: Line/column information for validation errors (when source JSON available)
• Full Type Support: All 34 primitive and compound types from JSON Structure Core v0

Technical Requirements

• Swift 5.9+ (for modern concurrency features)
• Swift Package Manager (SPM) for distribution
• Cross-platform: macOS, iOS, Linux, Windows
• Use Foundation.JSONSerialization or Codable for JSON parsing
• No Apple-only framework dependencies (pure Swift for portability)

Package Structure

swift/
 Package.swift
 README.md
 Sources/
    JSONStructure/
        SchemaValidator.swift
        InstanceValidator.swift
        Types.swift
        ErrorCodes.swift
        JsonSourceLocator.swift
        ValidationResult.swift
 Tests/
     JSONStructureTests/
         SchemaValidatorTests.swift
         InstanceValidatorTests.swift
         TestAssets.swift

CI/CD

• GitHub Actions workflow with macOS runner (required for Swift)
• Test matrix: Swift 5.9, 5.10 on macOS and Linux
• Integration with shared test-assets/ for cross-SDK validation

Reference Materials

• SDK Guidelines: SDK-GUIDELINES.md - Start here
• Error Messages: assets/error-messages.json
• Test Assets: test-assets/
• Meta-schemas: meta/
• Reference implementations: go/, rust/, typescript/

Acceptance Criteria

• All test-assets pass validation
• CI workflow passes on macOS and Linux
• README with installation and usage examples
• Conformance checklist completed (see SDK-GUIDELINES.md)
• Published to Swift Package Index (or ready for publication)