Composite Slots & the Flow Rename — One Toggle, Every Layout Mode

We thought the spec called for two patches and a rename. Halfway through the second patch the architecture broke in our face — a Split view with a working ✎/⊞ toggle but a stubbornly empty pane next to it — and we realized the toggle didn't belong inside either remote, it belonged to the slot. The fix turned into a shell-level concept that pays off in every layout mode.