docs: explain toJSON validation with anyOf and oneOf

[zanmlakar]

Document that anyOf and oneOf validate nested objects before calling their toJSON() methods. Properties supplied only by toJSON() therefore cannot satisfy the branch's required properties.

Add a README example showing explicit conversion, plus tests covering both keywords and the direct-object-schema comparison. This documents existing behavior; serialization semantics are unchanged.

Refs #621.

Verified locally on Linux with Node 26.7.0:

• 588 unit tests and 15 type assertions passed, with 100% configured coverage.
• All 10 focused toJSON tests passed.
• Lint, the README example, and all seven benchmark groups passed.

AI assistance: OpenAI Codex helped research the issue, prepare the documentation and tests, and run the listed checks.