Extends layoutStore with previewSession/commitPreview, mirroring
multiview.ts's preview controller: a throwaway tab for a session no pane
holds, byte-for-byte restore on end, and every real layout-changing action
drops an active preview first. Never schedules a save while a preview is up.
Adds usePreview, wrapping the store actions with previewSchedule.ts's
120ms leading-edge throttle, plus a touch guard and an immediate `end()`
for real changes. MultiView/TabBar render the preview hue on the pane
titlebar, canvas ring/chip, and the throwaway tab (dashed, unclickable).
Closes#249: layoutStore.ts wraps the existing layout.ts tree model
in Zustand, persisting to localStorage on a 400ms debounce and adding
boot reconciliation to prune leaves for sessions no longer live.
MultiView/TabBar/PaneContainer/PaneTitle render tabs, nested splits
with draggable dividers, drag-and-drop tab reordering, and dispatch
each pane to TerminalPane or StructuredPane by session view.
Preview mode (#251) and sidebar drag-to-dock stay out of scope here.
Miguel Palhas