feat: support OpenAPI 3.x specifications

[Himenon]

Summary

• Add OpenAPI 3.x validation support for 3.0.x, 3.1.x, and 3.2.x documents.
• Support OpenAPI 3.1 JSON Schema features including type arrays, boolean schemas, const, prefixItems, $defs, and $ref siblings.
• Support OpenAPI 3.2 features including querystring, QUERY, additionalOperations, itemSchema, and reusable components.mediaTypes.
• Add TDD coverage for validation and generated TypeScript output.
• Document OpenAPI-version-specific additions with Japanese code comments.

Validation

• vitest run --config ./vitest.config.ts — 47 tests passed
• tsc --noEmit — passed
• biome check . — passed
• depcruise --validate .dependency-cruiser.cjs src — passed
• tsup — passed

The existing snapshot suite still has the repository\x27s pre-existing generated-header mismatch (snapshots use v2.0.8 while package.json uses v2.0.10).

Closes #18