Implement FormDataEvent file and dirname entries #609

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

Complete the browser-shaped FormData event and remaining ordinary form-entry construction used by unchanged Code OSS.

Scope

• dedicated FormDataEvent in top-level and nested realms;
• required readonly FormData member and synchronous bubbling same-object dispatch;
• selected-file and empty-file entries;
• ordered dirname entries with bounded explicit, inherited, and automatic direction;
• preserve submit/formdata and document-tree ordering from Implement form submit methods and formdata ordering #608;
• focused browser/profile/deferred-validation contracts.

Deferred scope

Form-associated custom elements require ElementInternals, attachInternals, formAssociated, setFormValue, and lifecycle work. Navigation/encoding transport remains separate.

Validation truth

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