Function fast_hash 

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

Fast, non-cryptographic 64-bit content hash used only for in-memory change detection.

Uses xxh3: a SIMD-friendly hash that processes 64-byte blocks with parallel lanes, delivering near memory-bandwidth throughput. The value is never persisted or sent on the wire, so only the property that identical bytes hash identically matters. A collision between two distinct stripes is ~2^-64, and the next real content change or a requested keyframe repaints any missed update anyway.