Constant MAX_PERMUTE_VARS 

const MAX_PERMUTE_VARS: usize = 6;

Decode-side re-statement of the binder’s MAX_PERMUTE_VARS: PERMUTE expands to n! orderings, so a skewed or corrupt plan carrying an oversized one would allocate factorially before any row is processed. Every other malformed input to this decoder fails with a descriptive error; the sizes must too.