I heard from an authoritative source that the compiler is incomplete and the goal is to use a software implementation of 128-bit IEEE. I did some quick tests on the SQLite that came with my Mac Mini and Raspberry Pi4 and found that the expression "CAST(9223372036854774800 AS real)" displays as 9.22337203685477e+18 on the former and 9.22337203685478e+18 on the latter. The Pi shows the same rounding error that I get defining LONGDOUBLE_TYPE as double.