Constant PREPARATION_IMPL_VERSION 

pub const PREPARATION_IMPL_VERSION: u32 = 0;

The engine’s current preparation-implementation version — the single source of truth for “which preparation implementation is live”.

No preparation implementation exists yet, so this is 0 (“none”). It nonetheless participates in hash_binding: a future preparation implementation bumps this constant, which — because the preparation identifier + this version are both hashed — invalidates every prior finding keyed on the old hash(D) by construction.