Value set literals refuse array_in's dimension decoration ([1:1]={…}) and {{}} #128

Value set literals refuse two array literals PostgreSQL's array_in reads as a one-dimensional array: the dimension decoration [lower:upper]= and the nested empty {{}}. Loud (FormatException). Measured on main (3ecfa10) against PostgreSQL 17.10:

Int32Set.Parse("[1:1]={1}", null);     // FormatException: not a valid array literal
StringSet.Parse("[0:1]={a,b}", null);  // FormatException
Int32Set.Parse("{{}}", null);          // FormatException

SELECT '[1:1]={1}'::int[], array_lower('[0:1]={1,2}'::int[], 1), '{{}}'::int[];   -- {1} | 0 | {}

Every value set type. Found by LiteralCorpusProbeTests (#123), family dimensions. A real two-dimensional literal ({{1},{2}}) is refused on purpose and agrees.

Authority. docs/value-sets.md#parse-and-print-literals: "The grammar is PostgreSQL's array_in." (claim S12). array_out prints the decoration whenever the lower bound is not 1, so a column written by another tool ('[0:1]={1,2}') prints text the library cannot read back.

Options. Read the decoration and drop it (a set has no index), and read {{}} as empty; or document both as refused next to S12.