Implement form submit methods and formdata ordering #607

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

Complete the browser-shaped form submission method and event-ordering surface over the merged validation path.

Scope

• HTMLFormElement.submit() and requestSubmit(submitter);
• submitter TypeError/NotFoundError and form-owner checks;
• requestSubmit reuse of submit-attempt user-validity and constraint validation;
• uncanceled submit followed by FormData construction and formdata dispatch;
• canceled-submit cutoff and legacy submit validation/event bypass;
• FormData(form, submitter) event dispatch in main and frame realms;
• dedicated HTMLFieldSetElement validity IDL/wrapper;
• focused browser/profile/deferred-validation contracts.

Deferred scope

Navigation/encoding transport, FormDataEvent constructor, file/dirname/form-associated custom-element entries, interactive validation UI, and structural ancestor invalidation remain separate.

Validation truth

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