Warning row, "↑ N more" / "↓ N more" hints, and the items list were all
conditionally rendered, so the picker jumped in size as the selection
moved or providers without a warning slid into view.
Render every slot unconditionally: warning falls back to a blank line,
hints render an empty string when at the edge, and the items grid always
emits VISIBLE rows padded with blanks. Height is now constant across
providers, model counts, and scroll position.
c3b8c8e42c