Add form-associated custom element lifecycle and entries #611

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

Add the bounded form-associated custom-element lifecycle required for custom FormData entries.

Scope

• branded ElementInternals and HTMLElement.attachInternals;
• snapshot static formAssociated during custom-element definition;
• setFormValue(value,state) for string, File, FormData, and null;
• live form-owner and disabled lifecycle on connect, disconnect, form/id, and fieldset-disabled changes;
• reset callback and ordered custom entries in top-level/nested FormData;
• focused contracts/profile/deferred-validation documentation.

Validation truth

git diff --check passed. Builds, WPT, benchmarks and product acceptance remain unexecuted. Validity UI, restoration trigger, labels/ARIA/custom states, form.elements and structural invalidation remain separate.