listing: restore full-segment search load (find every match)

Revert the "search only the loaded portion" shortcut from f5aaf50 and go back to
a single, guarded background load_all before matching, so a search finds every
occurrence in the segment (not just whatever streamed in so far). The guard
(_search_loading + queued callbacks) shares ONE load across per-keystroke
updates instead of spawning/cancelling a worker each keypress.

The wedge that shortcut was meant to avoid turned out to be unrelated to the
search load: it is a pre-existing navigation bug (opening a function right after
scrolling to the bottom of another leaves the listing cursor out of range and
'/' fails to focus the search input, so typed chars leak into the view as
listing verbs, one of which is 'u'=undefine). Reproduces at 0c097f5, before any
of this work; tracked separately.

Verified in isolation: search 3.1s, matches found; search/mouse/func_banners/
listing_view/continuous_view/decomp_nav/follow_xrefs 45/0.

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