Implement ElementInternals constraint validation #615

Parent: #257. Tracking epics: #235 and #227.

Integrate form-associated custom elements into the merged native constraint-validation and submission model.

Scope

• ElementInternals setValidity and live branded ValidityState;
• validationMessage, willValidate, checkValidity, and reportValidity;
• host selector plus form/fieldset aggregation and invalid-event dispatch;
• submit/reset user-validity and bounded retained-state restoration hook;
• host-subtree recascade only, without structural ancestor invalidation;
• focused browser/profile/deferred-validation contracts.

Validation truth

git diff --cached --check passed. Interactive UI, ancestor recascade, labels/ARIA/CustomStateSet, form.elements, persisted restoration and runtime/product gates remain deferred.