A specification is a NUL-terminated string of six to eight characters:
frame type, state, left, right, top, bottom margins, optional radius and
optional width. Type uses digits then letters and must be below
FST_COUNT; state is zero or one. Margins and optional fields are
hexadecimal 0..f. The parser also accepts uppercase letters. Omitted
optional fields become zero after the first six fields parse.
Parsing is not transactional: invalid later characters can leave earlier
fields changed. NULL or invalid length returns before field updates.
This class ignores that status, so use valid input. Serialization uses
six characters when both optional values are zero, otherwise eight. Get
returns borrowed mutable storage valid until disposal and overwritten by
another Get; the resulting string may be passed to Set.