Expose form control constraint-validation APIs #603

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

Expose the browser-shaped constraint-validation API over the merged live form-state model. The API must aggregate required, text, numeric, date/time, radio, and select validity without duplicating selector or structural invalidation.

Scope

• generated dedicated prototypes for input, textarea, select, and button plus illegal-constructor ValidityState;
• live SameObject validity wrapper with all eleven flags;
• validationMessage, willValidate, and per-control custom error state;
• setCustomValidity, checkValidity, and reportValidity;
• cancelable, non-bubbling invalid dispatch;
• recascade :valid/:invalid only when custom-error validity changes;
• focused WPT-derived/browser, native invalidation, profile, and deferred-validation contracts.

Deferred scope

Form/fieldset aggregation, submission-time validation, interactive validation UI, datetime-local/file-input behavior, and UI-only bad-input/user-length transitions remain separate slices.

Validation truth

git diff --cached --check passed. Authored contracts, builds, WPT, benchmarks, product acceptance and computer-use remain unexecuted.