Extracts the app-launcher substance (desktop-entry parsing, fuzzy
matching/ranking, launch history, launching) out of breadbox into a
reusable ecosystem crate, so breadbar's future embedded capsule and
breadbox's overlay window can share one implementation instead of two
(THEME_SYSTEM_PLAN.md Phase 4/6a). The GTK4 results-list widget lives
behind a `gtk` feature, mirroring bread-theme's `gtk`/`adw` gating, so a
headless consumer isn't forced to link GTK.
Adds unit tests for fuzzy_score/matches_term/priority_rank, which were
previously untested pure functions inside breadbox's main.rs.
