syncthing-0001: deviceDownloadState.blockIndexes []int with slices.Contains O(B)
called per device per block in file sync hot path. Fix: map[int]struct{}.
UNDF-2026-000000850. 2.9x measured at B=1000.
nextcloud-server: 386 in_array() calls, all on constant-size or config-level arrays.
gitea: 91 slices.Contains, all on small lists; container.Set used for hot paths.
