[WaylandYang]

Fixes #1463. Corrects the comments behind #1462.

• Overview deadline flushes a fast key before its settle window #1463: when the overview deadline fired it flushed the fast coalescer, so keys still inside their 300 ms window went out early. It now only pushes into the fast layer and rides the open window. If no fast window is open, the overview lands up to SETTLE_MS later (3.3 s instead of 3 s). Reconnect and unmount still go through flushAll, so prefix folding is unchanged.
• Sustained event burst invalidates the same key twice #1462: no behaviour change. The window starts at the first event and does not reset, as While a source pushes, the graph overview recounts the base every few seconds instead of three times a second #1311 chose, so a steady push still refreshes periodically. The comments said "silence", which reads as a debounce, so they now describe the window.

Test: the overview deadline rides the open fast window instead of flushing it early. It fails with the old flush. The existing overview timing test now allows for the extra SETTLE_MS.

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