Prefix Is Not Identity

A cache or dedup key derived from a fixed-length prefix of a value, on an implicit assumption that the prefix carries per-actor entropy. When the producer's encoding concentrates entropy past the prefix window (a JWT payload past the header, a URL path past the host, a salted hash past the salt), every actor's value maps to one bucket. The type signature is `str`; the entropy contract is documentation. When the producer's shape drifts, the slice goes from fingerprint to constant without warning.