CI slice 1/12 caught a regression in _mask_control_split_tokens: a
COMPLETE prefix token at end-of-line followed by ordinary text (browser
accessibility annotations: 'ghp_<tok>\nbutton [ref=e3]: Copy') was
joined across the newline into one stripped-copy match, and the mask
swallowed the adjacent line ('button' disappeared).
Join only when no fragment inside the span matches _PREFIX_RE on its
own — a self-matching fragment is already handled by the ordinary
prefix pass, so joining can only cause damage. All smuggling shapes
(ESC/ZWSP/newline splits with under-length fragments) still mask;
regression test added and mutation-checked (fails without the guard).
