docs: Add refactor design for OpenAPI 3.1 readiness

Captures the locked decisions from brainstorming: sealed Schema hierarchy,
consumer-supplied Map<String,Object> spec parsing, RFC 7807 problem-detail
validation errors, single DefaultValidator with pattern-match dispatch, Java
25 target. Includes a prioritized 30-item gap inventory for follow-up specs.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: thced