Constant SCRATCH_ARENA_BYTES 

pub const SCRATCH_ARENA_BYTES: u32 = _; // 65_536u32

Bytes reserved for the scratch arena. Sized to comfortably hold the args+results of a multi-arg primitive function (each arg is at most 8 bytes; 64 KiB is enough for ~4000 i64 args). Operators tune via add_jit_dispatch_to_linker_with in test harnesses.