A parsed form body: the read-only view IRequest.formData?() resolves and
parseFormBody returns.
Named FormBody, never FormData, so it cannot shadow the global. The web
standard's semantics are adopted; the write side (set/append/delete)
is not — a request body is a fact about the request, not a collection a
handler edits.
Returns the FIRST value for a name, or undefined when the name is
absent — the web standard's get, with undefined (narrowable) in place
of null.
Returns every value for a name in wire order — the web standard's
getAll, so a repeated field (multi-select, multi-file) keeps the order
the client sent.
The returned array is the parse's own READ view, not a defensive copy:
mutating it mutates what later getAll calls return. Treat it as
read-only (the IResponse.snapshot() precedent — a copy would allocate
on every read of a value most readers never touch).