vtabs: terminal with vertical tabs on libghostty-vt + GTK4

Uses libghostty-vt (the API Ghostty documents for external embedders) for
the terminal core. Ghostty's other C API, ghostty.h, exposes a full terminal
surface but only supports macOS and iOS platform tags, so it cannot be
embedded on Linux.

We supply the layers libghostty-vt deliberately leaves out: PTY and process
management, a Cairo/Pango cell renderer, and a GTK4/libadwaita UI with a
Zen-style vertical tab sidebar.

Nix pins the whole toolchain (Zig 0.16 via zig-overlay, GTK 4.22, libadwaita)
so no host setup is needed.

README.md

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+# vtabs
+
+A proof-of-concept terminal emulator with **vertical tabs**, built on
+[libghostty-vt](https://github.com/ghostty-org/ghostty) and GTK4/libadwaita.
+
+The sidebar holds the window controls, a new-tab button, and one row per tab —
+the layout Zen Browser uses for vertical tabs — with the terminal inset to its
+right.
+
+## Quick start
+
+```sh
+nix develop          # Zig 0.16 + GTK4 + libadwaita, no host toolchain needed
+zig build run
+```
+
+Everything is pinned by `flake.nix`; nothing needs to be installed on the host.
+
+## Which "libghostty" this uses, and why
+
+Ghostty ships two different C APIs, and only one of them is usable here:
+
+| | `include/ghostty.h` | `include/ghostty/vt.h` |
+|---|---|---|
+| name | "libghostty-internal" | **libghostty-vt** |
+| scope | whole terminal incl. renderer | VT core only |
+| platforms | `GHOSTTY_PLATFORM_MACOS`, `GHOSTTY_PLATFORM_IOS` | any |
+| intended for | Ghostty's own macOS app | external embedders |
+
+`ghostty.h` is the one that would hand you a ready-made terminal surface, but
+it has no Linux platform tag at all — its `ghostty_platform_e` enum only knows
+macOS and iOS, and its own header says it is "tailored to the needs of the
+macOS app". On Linux, Ghostty does not embed itself through that API; it builds
+its GTK apprt directly into the binary.
+
+So this project uses **libghostty-vt**, the API Ghostty documents for external
+embedders. That means we get, for free and battle-tested:
+
+- escape sequence parsing and full terminal state
+- screen, scrollback, line wrapping, and reflow on resize
+- key and mouse event encoding (including the Kitty keyboard protocol)
+- paste safety checking and bracketed paste encoding
+
+and we supply everything above it: process management, rendering, and the UI.
+
+We consume it as a **Zig module** rather than through the C ABI. Ghostty's
+`build.zig` detects that it is being used as a dependency and, in that mode,
+builds only libghostty-vt — no GTK, no executable — so `zig build` pulls in
+just the VT core.
+
+## Architecture
+
+```
+main.zig       AdwApplication, CSS loading
+Window.zig     sidebar + GtkStack of terminals, tab management, shortcuts
+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
+key.zig        GDK keyval -> libghostty-vt key mapping
+theme.zig      colors libghostty-vt has no opinion about
+```
+
+Two design choices worth calling out:
+
+**No IO thread.** The PTY is read on the GLib main loop through a unix fd
+watch (`g_unix_fd_add`). Terminal state is therefore only ever touched from the
+main thread, so the renderer reads the screen with no locking. Ghostty itself
+uses a dedicated IO thread; that is the right answer for a real terminal, but
+this is dramatically simpler and is not a bottleneck at interactive speeds.
+
+**No GPU renderer.** Ghostty rasterizes glyphs into an atlas and draws on the
+GPU. Here, each frame walks the visible rows, groups cells into runs of
+identical style, and hands each run to Pango. That is far more work per frame
+in principle, but a terminal grid is small.
+
+## What works
+
+- Real shell on a real PTY, with a controlling terminal so job control,
+  Ctrl-C and SIGWINCH behave
+- Full SGR rendering: 16/256/true color, bold, italic, underline,
+  strikethrough, inverse, and the bright-on-bold convention
+- 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
+
+### Shortcuts
+
+| | |
+|---|---|
+| `Ctrl+Shift+T` | new tab |
+| `Ctrl+Shift+W` | close tab |
+| `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 |
+
+## Not implemented
+
+This is a proof of concept, and the following are deliberately absent:
+
+- **Mouse selection and copy.** libghostty-vt provides the selection and
+  mouse-encoding primitives, but nothing here is wired to them yet, so there is
+  no way to copy text. Paste works.
+- **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.**
+- **Custom terminfo.** `TERM` is reported as `xterm-256color` rather than
+  `ghostty`, since we don't install a terminfo entry.
+
+## Development
+
+`./b.sh` wraps `nix develop --command zig build` and strips the enormous
+command line Zig prints on failure.
+
+`./shot.sh out.png "text to type"` runs the app inside a throwaway **headless
+Sway** and screenshots it. This keeps UI checks entirely out of your real
+Wayland session — nothing appears on screen, and it works while the session is
+locked.
+
+Screenshots use `$SHELL` like the real app does; set
+`VTABS_SHOT_SHELL=/bin/sh` for output uncluttered by your shell's rc files.
+
+One caveat when driving it: `wtype` loses the first keystroke of every
+invocation while the compositor adopts its freshly uploaded keymap, so scripted
+input should begin with a throwaway key. That is a quirk of the injection tool,
+not of the terminal.