- BUG-085: ring buffer EventsSince off-by-one — change < to <= so
afterSeq == oldestSeq returns nil (triggers full ready payload)
- BUG-087: GracefulStop not idempotent — wrap body in sync.Once to
prevent double lkProcess.Stop() on concurrent calls
- BUG-090: FTS query truncation at byte boundary — use []rune
truncation to preserve valid UTF-8 for CJK/emoji input
- BUG-091: updater downloadFile double-closes file on Windows —
add closed sentinel to guard defer against explicit Close()