Swallow-by-default mouse handler for placed secondary overlays (#2281).

The eight footer-band overlays (float popup, agent context, tool manager, extension panel, observatory, edit timeline, notifications, extension overlay) are now FocusTree nodes with a real rect. A FocusTree node with no handler bubbles its mouse events to ancestors (router passthrough semantics), which would let a click or scroll over a visible overlay reach the buffer underneath and move its cursor. That is the exact bug the epic's AC-2 forbids.

This handler is the minimal safety floor: it consumes (:handled) presses, releases, drags, and scroll wheel events that land inside the overlay's rect, so they never bubble to the buffer. It does not yet interpret them, per-surface activation semantics (clicking a notification, an observatory row, a timeline entry) land as the epic's follow-up children (#2330). Those children replace this sink with a surface-specific handler; until then, the overlay is safe to click without editing the buffer behind it.