Function logos_narrow_i128 

pub fn logos_narrow_i128(v: i128) -> i64

Narrow a width-bounded i128 chain result back to i64 (overflow ruling v2, stage 2): the codegen lowers an exact chain whose worst-case bit-width provably fits i128 as native i128 arithmetic (every intermediate exact by construction) and narrows ONCE at the root. Same canonical error text as expect_i64 — an i128 prints the same digits LogosInt would.