D2 — R-695: single-flight file-browser sync; no empty kept folder listed or bound

Tests TestR695_QueuedSyncsCoalesceIntoOne (6 requests → 2 syncs; a later request is never dropped) and TestR695_AnEmptyDatedKeptFolderIsNeverListed; both red-proofed (RP-r695.txt). Not reproduced live (the original was a same-second race).