[v0.4] Add split window layout tree

[owainlewis]

Goal: Add split-screen editing with a small owned layout tree.

Context: Cortex should support the Emacs-shaped split workflow before the embedded terminal pane.
Splits should show buffers in separate windows with independent point and scroll where needed.
This is the big visual/workflow step, so it should build on the buffer-list work rather than inventing a second model.

Proposed approach:

• Introduce a window/layout tree with internal split nodes and leaf editor windows.
• Each leaf window references a buffer and owns its own view state.
• Render each leaf into its own rectangle through the existing renderer boundary.
• Add commands for split-window-below, split-window-right, delete-window, delete-other-windows, and other-window.
• Bind C-x 2, C-x 3, C-x 0, C-x 1, and C-x o.
• Keep modeline/status treatment clear for focused and unfocused windows.
• Preserve cursor placement in the focused window.

Acceptance criteria:

• C-x 2 creates a horizontal split below the current window.
• C-x 3 creates a vertical split to the right of the current window.
• C-x o moves focus between windows.
• C-x 0 deletes the focused window when more than one window exists.
• C-x 1 deletes other windows and keeps the focused window.
• Each window keeps its own point and scroll state.
• Saving saves the buffer shown in the focused window.
• Small terminal sizes and long lines do not panic.
• Existing single-window behavior still works.

Verification:

• Run cargo test.
• Manually split vertically and horizontally, move focus, edit, save, close splits, and quit.
• Manually check small terminal sizes if practical.
• Manually confirm terminal cleanup after exit.

Out of scope:

• Tabs.
• Embedded terminal panes.
• Arbitrary resize commands.
• Mouse support.
• Dragging split dividers.
• Tree or sidebar UI.