pub const PREFIX_LEN: usize = 40;Expand description
Family-key length: short enough to catch “same problem, different wording”,
long enough to distinguish unrelated tickets. Bumped down from 80 after a
real friction pair collided at the semicolon (queue 324 test fixture).
pub since q328 SIGNCHECK so sign_check reuses the exact same family key
(single vocabulary, no rival contradiction concept).