Function fnv1a 

pub fn fnv1a(bytes: &[u8]) -> u64

Hashes bytes with FNV-1a (64-bit).

core::hash::Hasher/std::hash::DefaultHasher are either the wrong shape (no portable digest guarantee) or unavailable at all under no_std, so Observable implementors get a small, dependency-free digest instead. Not cryptographic, and not guaranteed stable across retroglyph-core versions: only ever compared within a single test run, never persisted.