fix: readOnly semantics and React 18 className mapping

[thedhanawada]

Summary

• Expose the boolean readOnly prop and element property in public declarations.
• Map readOnly to readonly attribute presence consistently during SSR, lazy upgrade, and updates.
• Map className to class explicitly: React 18 otherwise emits classname/className and loses CSS classes.
• Document interface >=1.8.0 for read-only editing; retain existing peer compatibility.

Validation

• Full npm test: 37/37 on React 18 and 37/37 on React 19 with published interface 1.8.1/core 2.5.5.
• Bundler and NodeNext declaration checks pass on both React versions.
• Cold/warm true/false/omitted readOnly + reactive toggles, SSR boolean attributes, class updates/removal, existing callbacks and StrictMode tests.
• Before the class fix, React 18 failed two existing class mapping DOM/SSR assertions; both now pass.

Release held pending integrated feature validation and publishing.