feat(native): split layouts as window tabs, with delegates beside the orchestrator

[omartelo]

What

Split layouts for the native window, designed from scratch rather than ported from the web's walls.

• Layouts are tabs in the topbar strip, after the projects, and can mix projects. Rename one by double-clicking it or from its menu; a layout down to one pane closes.
• Free split tree. Split the focused session right (Ctrl+Shift+G) or below (Ctrl+Shift+I) with a session picker, or drag a card from the sidebar onto a pane's edge. Drag a pane by its header onto another pane's edge to move it, or onto its middle to swap the two; Ctrl+Shift+R turns a pair between side by side and stacked. Drag the gap to resize, zoom the focused pane (Ctrl+Shift+M), cycle panes (Ctrl+Shift+F), take one out (Ctrl+Shift+W).
• Dragging feels like a browser. A dragged card leaves the sidebar as a small pill under the pointer; resting it on another tab for 0.6 s brings that tab up. Drop zones only offer what fits: an edge whose pane would drop under the 480dp floor is not offered, and swap is only between panes of the layout.
• Pane header: status, the project as an outlined pill only when the layout mixes projects, the session label, and the worktree only when the session runs in one.
• Delegates beside the orchestrator (second commit). A session that opened others, in any project (originSessionId), offers them in the footer while it is in no layout: 3 delegates · Show beside opens a layout with it at the left and its delegates stacked at the right, as many as fit. In a layout its pane header reads 2 of 3 here · Add 1. A delegate in another layout is moved only after a confirm. A card in a layout names it.

How

• layouts.go is the pure tree (paneNode, split/remove/move/rotate/stack), persisted under lich.native.layouts; the web's lich.panes is untouched and neither window reads the other's.
• A session sits in one pane of one layout (one PTY, one size). Clicking its card shows it in its layout; its project tab shows it alone.
• Drop zones are worked out from the pointer read by one handler over the whole window and where panes were drawn, not from Gio transfer targets: Enter reached targets laid out mid-drag only some of the time.
• ui.SortCarry gains Hold (Gio dereferences a forgotten drag source after a tab switch takes the list off screen) and Release; SortList.Leaves lets a card leave the list without the list reshuffling.
• The delegates room check (columnRoom) counts panes against the 180dp floor from the last frame's rects.

Notes

• docs/ceilings.md gains a bullet on the native layouts: pane floor, exit band and file drop only on the focused pane, pinned cards can't be dragged, zones outside the carry, Hold, the four native-only hotkeys that a web rebinding drops, and the delegates footer item living outside the Footer layout.
• All new strings land in the 7 native locales.

Verification

• native/: gofmt -l ., go vet ./..., go test ./... (new tests for the tree, the model routing, drop zones, SortCarry, delegates).
• Exercised in a disposable backend on Xvfb with screenshots: split, drag, swap, rotate, tab dwell, Show beside, Add, the move confirm.
• Reviewed by the maintainer in a real window.