Multiple terminals per tab, rearrangeable by drag or keyboard

A tab is now a view holding an ordered list of panes rather than a single
terminal. New panes open side by side; moving one against a top or bottom
edge restacks the view, and against a side edge returns it to a row.

Layout is a flat list with one orientation per view rather than a split
tree. That covers a shell beside an agent without the structure a tree
needs, and it can be replaced once mixed layouts actually matter.

Panes live in a GtkBox and are reordered in place, so rearranging never
unparents a terminal and running processes and scrollback survive the move.

Dragging a pane uses its header as the handle so it never competes with the
terminal's own mouse handling, and drops go through the same moveRelative
path as the Ctrl+Shift+arrow shortcuts.

Also fixes an ordering bug this surfaced: View.create used to add its first
pane immediately, firing the title callback into a Tab that had not been
initialized yet. The view is now created empty and the caller adds the pane
once it has finished wiring itself up.

README.md

@@ -77,7 +77,9 @@ just the VT core.
 
 ```
 main.zig       AdwApplication, CSS loading
-Window.zig     sidebar + GtkStack of terminals, tab management, shortcuts
+Window.zig     sidebar + GtkStack of views, tab management, shortcuts
+View.zig       one tab's content: an ordered list of panes + their layout
+Pane.zig       a terminal plus its header, drag source, and drop target
 Terminal.zig   GtkDrawingArea: Cairo/Pango renderer + input handling
 Session.zig    libghostty-vt Terminal + parser, fed by the PTY
 Pty.zig        openpt/fork/exec, controlling terminal setup
@@ -85,6 +87,20 @@ key.zig        GDK keyval -> libghostty-vt key mapping
 theme.zig      colors libghostty-vt has no opinion about
 ```
 
+A tab is a **view**, and a view holds one or more terminal **panes**. New panes
+open side by side; moving a pane to a top or bottom edge restacks the view, and
+moving it to a side edge puts it back in a row.
+
+**Layout is a flat list, not a split tree.** A view has one orientation shared
+by all its panes, so it can express "three side by side" or "three stacked" but
+not "two beside a stack of three". A tree would handle the general case and is
+where this goes if mixed layouts turn out to matter; it just brings a lot of
+structure that a shell-next-to-an-agent view doesn't need yet.
+
+Panes are held in a `GtkBox`, which can reorder its children in place. Nothing
+is ever unparented, so rearranging a view never disturbs the running
+terminals — their scrollback and processes carry straight through the move.
+
 Two design choices worth calling out:
 
 **No IO thread.** The PTY is read on the GLib main loop through a unix fd
@@ -108,20 +124,29 @@ in principle, but a terminal grid is small.
 - Block / bar / underline / hollow cursor styles
 - Scrollback via mouse wheel; typing snaps back to the prompt
 - Resize reflows the grid and notifies the child
-- Window title (OSC 0/2) becomes the tab label
-- Tabs: create, close, switch; closing the last one closes the window;
-  a child exiting closes its own tab
+- Window title (OSC 0/2) becomes the pane header and tab label; the tab shows
+  its pane count once a view holds more than one
+- Tabs: create, close, switch; closing the last one closes the window
+- Multiple terminals per tab, rearranged by keyboard or by dragging a pane's
+  header; closing the last pane in a view closes its tab
 
 ### Shortcuts
 
 | | |
 |---|---|
 | `Ctrl+Shift+T` | new tab |
-| `Ctrl+Shift+W` | close tab |
+| `Ctrl+Shift+E` | new terminal in the current tab |
+| `Ctrl+Shift+W` | close the focused terminal (closes the tab with its last one) |
+| `Ctrl+Shift+←/→/↑/↓` | move the focused terminal within its view |
 | `Ctrl+Shift+V` | paste (bracketed-paste aware, refuses unsafe pastes) |
 | `Ctrl+PageUp/PageDown` | previous / next tab |
 | `Alt+1`..`Alt+8` | jump to tab N, `Alt+9` jumps to the last |
 
+Dragging a pane by its header does the same thing as the arrow shortcuts: the
+edge you drop against decides both the order and the view's orientation. The
+header is the drag handle rather than the whole pane so that dragging never
+competes with the terminal's own mouse handling.
+
 ## Not implemented
 
 This is a proof of concept, and the following are deliberately absent:
@@ -132,7 +157,11 @@ This is a proof of concept, and the following are deliberately absent:
 - **Ligatures and complex shaping.** Each run is drawn independently at a
   fixed grid offset, so text that needs shaping across cell boundaries won't
   look right.
-- **Kitty graphics, hyperlinks, tab reordering, split panes, config file.**
+- **Resizable splits.** Panes in a view divide the space evenly; there are no
+  draggable dividers. Moving to `GtkPaned` would add them.
+- **Mixed layouts and moving panes between tabs.** See the flat-list note
+  above; panes can only be rearranged within their own view.
+- **Kitty graphics, hyperlinks, tab reordering, config file.**
 - **Custom terminfo.** `TERM` is reported as `xterm-256color` rather than
   `ghostty`, since we don't install a terminfo entry.