Fix CLI reference default serialization

[muxx]

Summary

• serialize structured Commander defaults as deterministic JSON
• omit empty object defaults used as repeatable option accumulators
• reject [object Object] in the generated CLI reference contract
• add focused unit coverage for primitive, array, object, and empty-object defaults

Verification

• make generate is deterministic
• pnpm format:check
• pnpm lint
• pnpm typecheck
• pnpm test
• release artifact build, validation, and installation checks
• downstream docs materialization and desktop/mobile visual checks