LT Hash — the summation hash that keeps app-state integrity across mutations. Ported from Baileys' pre-WASM pure-JS impl (src/Utils/lt-hash.ts before the rust bridge). Adding/removing a mutation gives the same hash as if the whole sequence had been applied in order, so patches can be verified incrementally.

The hash is 128 bytes = 64 little-endian unsigned 16-bit words. Each mutation mac is expanded with HKDF(mac, 128, salt="", info="WhatsApp Patch Integrity") into the same 64-word space and added/subtracted pointwise mod 2^16 (wraparound).

subtract_then_add/3 is the operation app-state uses: drop the macs of removed records, add the macs of new ones.