[v0.4] Add tabs over window layouts

[owainlewis]

Goal: Add tabs as saved window layouts, not as another buffer abstraction.

Context: The PRD treats tabs as groups of split layouts.
This should land after the split window layout tree so tabs can swap whole layouts instead of special-casing editor state.

Proposed approach:

• Add a tab list where each tab owns one window layout tree.
• Keep buffers shared across tabs through the editor-level buffer list.
• Add commands for tab-new, tab-next, and tab-close.
• Bind C-x t 2, C-x t o, and C-x t 0 if the keymap supports those sequences cleanly.
• Render a restrained tab indicator or modeline/tabline treatment.
• Preserve dirty state, save behavior, and focused window state across tab switches.

Acceptance criteria:

• Creating a new tab creates a fresh layout while keeping existing buffers available.
• Switching tabs restores the previous layout and focused window for that tab.
• Closing a tab removes that layout without deleting buffers or unsaved edits.
• The last tab cannot be closed into an invalid empty editor state.
• Dirty buffers remain protected on quit even when they are not visible in the active tab.
• Existing split behavior still works inside each tab.

Verification:

• Run cargo test.
• Manually create tabs, split windows inside a tab, switch tabs, close tabs, save dirty buffers, and quit.
• Manually confirm terminal cleanup after exit.

Out of scope:

• Named tabs.
• Persisted sessions.
• Dragging or reordering tabs.
• Tab-specific working directories.
• Embedded terminal panes.