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WHATWG FormData (spec subset, no deps; multipart serialization
studied from the read-only llrt reference). append/set/get/
getAll/has/delete/keys/values/entries/forEach; string
or Blob/File-ish values. entries/keys/values return arrays
(iterable: for..of fd.entries(), spread, Array.from, forEach);
[Symbol.iterator] is the entries array. fetch with a FormData
body serializes multipart/form-data in-binding (no core change).