The tail-header cap fix pushed SearchBlock's head/tail slicing arithmetic and
its copy-feedback hook over the duplication gate's threshold against the
byte-identical logic in TerminalBlock. Extract both into head-tail-cap.ts
(headTailCap) and use-copy-feedback.ts (useCopyFeedback) and consume them from
both blocks, deleting the clone rather than nudging it under the limit.
