Constant UNCHANGED_FOLD_NAMESPACE 

pub const UNCHANGED_FOLD_NAMESPACE: u64 = 0xD1FF_0002_0000_0000; // 15_131_813_281_578_090_496u64

D-fix.5: namespace for per-buffer unchanged-fold provider ids. Distinct high bits from HUNK_FOLD_NAMESPACE so a buffer’s hunk fold source and its unchanged fold source register under different overlay ids (both OR in the buffer’s low-32 id) — they coexist on the same buffer covering disjoint regions (hunks vs. the gaps between).