Pure, refactor-independent helpers for type-to-filter search in the
curses single-/radio-select menus: subsequence matching, filtered-index
mapping, cursor reconciliation, scroll clamping, and an active-search
key handler, plus unit tests.
Salvaged from #22758 (the curses event loop was since refactored into a
shared driver on main, so the integration is rebuilt in a follow-up
commit; these pure helpers and their tests carry over unchanged).
