Ordered Structured Fields data (RFC 9651 Section 3).
:type is :item, :inner_list, :list, or :dictionary. An item's :value
is a tagged bare item: {:integer, n}, {:decimal, {coefficient, scale}},
{:string, ascii}, {:token, ascii}, {:bytes, binary}, {:boolean, boolean},
{:date, unix_seconds}, or {:display_string, utf8}. Decimals are exact base-10
values, coefficient / 10^scale; floats are not accepted. Serialization rounds
to three places with ties to even. Scale is bounded to 18 and coefficient
magnitude to 10^30 - 1 before arithmetic.
Lists and inner lists contain ordered Values. Dictionaries contain ordered
{key, Value} pairs. Parameters are ordered {key, bare_item} pairs on items
and inner lists only. Parsing duplicates keeps the first key position and the
last value, as the RFC requires. Hand-constructed duplicates reject during
serialization. Inner lists cannot nest. Inspection omits values; access them
explicitly. This structure establishes no authentication or authorization.