Rip out Bonsplit and replace it with a direct AppKit split host

[lawrencecchen]

The current split path has too many geometry owners.

• SwiftUI owns pane anchors and sidebar, tab bar, and titlebar layout.
• GhosttyTerminalView is only a placeholder anchor.
• The real terminal view is reparented through TerminalWindowPortal.
• WindowTerminalPortal often reconciles geometry on a later main-queue turn.

That is why sidebar, titlebar, and tab bar changes can land a frame before the terminal. We can still reproduce this after targeted fixes.

This issue is not the broader paper-window-manager work in #1221. The goal here is narrower: keep today's split semantics, but replace Bonsplit and the portal-heavy split hosting path with a direct AppKit host, using the same basic ownership model that feels good in NiriCanvasView.

References:

• Sources/NiriCanvasView.swift
• Sources/GhosttyTerminalView.swift
• Sources/TerminalWindowPortal.swift

Scope:

• Rip out bonsplit for workspace pane layout.
• Reimplement the current split tree behavior in AppKit.
• Keep sidebar, workspaces, tabs-in-pane, browser panels, markdown panels, session restore, and keyboard shortcuts.

Core architecture:

• Workspace stays the source of truth for pane membership, selected panel, focused pane, split tree, and persistence.
• Add an AppKit WorkspaceSplitHostView or equivalent that owns the visible split tree directly.
• Each pane gets a real NSView container. The active panel view is mounted directly inside that container.
• For terminals, mount TerminalSurface.hostedView directly in the pane container, like NiriCanvasView does.
• Do not use GhosttyTerminalView as a placeholder anchor for the steady-state workspace split path.
• Remove TerminalWindowPortal from the hot path for workspace terminal panes. If some overlay still needs a portal, keep it out of geometry ownership.
• Preserve Ghostty's native macOS renderer path. Do not introduce MTKView, custom display links, or extra draw loops as part of this refactor.

Performance requirements:

• One layout owner per pane. Chrome and terminal geometry must update in the same AppKit layout pass.
• No steady-state geometry reconciliation that waits for an extra DispatchQueue.main.async.
• No terminal reparenting during normal split open, close, resize, sidebar toggle, or workspace switch.
• No sleep or timer based synchronization in app or runtime code.
• Keep the upstream Ghostty IOSurface presentation path intact.
• Steady-state CPU and GPU cost must be no worse than current normal rendering.

Acceptance:

• Cmd+B never shows sidebar, titlebar, or tab bar ahead of the terminal.
• Split divider drag never shows stale gutters or stale-width terminal frames.
• Opening or closing a split is visually atomic.
• Workspace switching does not show old terminal frames on the new workspace.
• Focus, IME, selection, drag and drop, copy mode, and search overlay continue to work.
• Session persistence round-trips the new split tree.
• Browser and markdown panels keep working inside the same split engine.

Implementation notes:

• Start from the direct-hosting pattern in NiriCanvasView, but implement the existing Bonsplit tree semantics instead of the scrollable canvas.
• Upstream Ghostty on macOS already has a good renderer and presentation path. The bigger problem here is view ownership and geometry timing, not switching Metal APIs.
• The refactor should reduce, not increase, the number of abstraction layers between split geometry and the terminal view.
• Land this behind a temporary hidden flag if needed, but the target should be full replacement, not a permanent second split engine.