The sync before-unload checkpoint staged renderer state and then queued
store.flushPendingAsync() fire-and-forget, so reload/restart/update paths
navigated while the staged session, scrollback and UI state were still
only in memory. Quit is covered by the will-quit flush barrier; those
paths were not.
Keep staging synchronous (no sync durable writes), but record the flush
outcome and expose it on app:await-before-unload-checkpoint. Restart,
updater install and lazy-chunk recovery reload now join that write before
navigating and abort the attempt when it fails or outlives a 20s deadline.
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