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.

shot.sh

@@ -0,0 +1,117 @@
+#!/usr/bin/env bash
+# Run vtabs inside a throwaway headless Sway and screenshot it.
+#
+# This keeps UI checks out of the developer's real Wayland session: nothing
+# pops up on screen, and it works even while the session is locked.
+#
+# Usage: ./shot.sh OUT.png ["keys to type"] [settle-seconds]
+set -u
+
+cd "$(dirname "$0")"
+OUT="${1:-/tmp/vtabs-shot.png}"
+KEYS="${2:-}"
+SETTLE="${3:-2}"
+
+RUNDIR="$(mktemp -d)"
+trap 'rm -rf "$RUNDIR"' EXIT
+
+cat > "$RUNDIR/sway.conf" <<CONF
+output HEADLESS-1 resolution 1200x800
+default_border none
+exec "'$RUNDIR/launch.sh' >'$RUNDIR/app.log' 2>&1"
+CONF
+
+# The nix dev shell exports bash functions into the environment. Any /bin/sh
+# the terminal's child spawns tries to import them and spews parse errors that
+# have nothing to do with vtabs, so drop them just before exec.
+cat > "$RUNDIR/launch.sh" <<'LAUNCH'
+#!/usr/bin/env bash
+while read -r fn; do unset -f "$fn" 2>/dev/null; done < <(declare -Fx | awk '{print $3}')
+exec "$VTABS_BIN"
+LAUNCH
+chmod +x "$RUNDIR/launch.sh"
+export VTABS_BIN="$PWD/zig-out/bin/vtabs"
+
+export WLR_BACKENDS=headless
+export WLR_LIBINPUT_NO_DEVICES=1
+# No GPU is available to a headless compositor here, so force software
+# rendering on both the compositor and GTK sides.
+export WLR_RENDERER=pixman
+export LIBGL_ALWAYS_SOFTWARE=1
+export GSK_RENDERER=cairo
+export XDG_RUNTIME_DIR="${XDG_RUNTIME_DIR:-/run/user/$(id -u)}"
+# The nix dev shell points SHELL at a minimal bash (no readline, no
+# programmable completion), which makes screenshots look broken for reasons
+# that have nothing to do with vtabs. Use the system shell instead.
+export SHELL="${VTABS_SHOT_SHELL:-/bin/bash}"
+export SWAYSOCK="$RUNDIR/sway.sock"
+# Don't let the app fall back to the real session.
+unset DISPLAY
+unset WAYLAND_DISPLAY
+
+for sock in "$XDG_RUNTIME_DIR"/wayland-*; do
+  [ -S "$sock" ] && basename "$sock"
+done > "$RUNDIR/pre-sockets" 2>/dev/null || true
+
+sway -c "$RUNDIR/sway.conf" >"$RUNDIR/sway.log" 2>&1 &
+SWAY_PID=$!
+
+# Wait for the compositor socket before talking to it.
+for _ in $(seq 1 50); do
+  [ -S "$SWAYSOCK" ] && break
+  sleep 0.2
+done
+
+# Sway picks its own socket name, so discover it by diffing the sockets in
+# XDG_RUNTIME_DIR against the ones that existed before we started.
+DISPLAY_NAME=""
+for _ in $(seq 1 50); do
+  for sock in "$XDG_RUNTIME_DIR"/wayland-*; do
+    case "$sock" in *.lock) continue;; esac
+    [ -S "$sock" ] || continue
+    name="$(basename "$sock")"
+    grep -qx "$name" "$RUNDIR/pre-sockets" && continue
+    DISPLAY_NAME="$name"
+    break
+  done
+  [ -n "$DISPLAY_NAME" ] && break
+  sleep 0.2
+done
+if [ -z "$DISPLAY_NAME" ]; then
+  echo "could not determine wayland display" >&2
+  cat "$RUNDIR/sway.log" >&2
+  exit 1
+fi
+export WAYLAND_DISPLAY="$DISPLAY_NAME"
+
+sleep "$SETTLE"
+
+if [ -n "$KEYS" ]; then
+  # wtype drops the first keystroke of every invocation while the compositor
+  # takes up its freshly uploaded keymap, so burn one on a harmless key.
+  wtype -k Shift_L 2>/dev/null || true
+  sleep 0.3
+  wtype "$KEYS" 2>/dev/null || true
+  sleep 1.5
+fi
+
+if [ -n "${VTABS_SHOT_DEBUG:-}" ]; then
+  echo "--- swaymsg get_tree (apps) ---" >&2
+  swaymsg -t get_tree 2>&1 | grep -E '"(app_id|name|pid)"' | head -30 >&2
+  echo "--- sway log ---" >&2
+  cat "$RUNDIR/sway.log" >&2
+  echo "--- app log ---" >&2
+  cat "$RUNDIR/app.log" >&2 2>/dev/null || echo "(no app log)" >&2
+fi
+
+if ! grim -o HEADLESS-1 "$OUT"; then
+  echo "grim -o failed, trying full capture" >&2
+  grim "$OUT" || echo "grim failed entirely" >&2
+fi
+
+swaymsg exit >/dev/null 2>&1 || kill "$SWAY_PID" 2>/dev/null
+wait "$SWAY_PID" 2>/dev/null
+
+echo "wrote $OUT"
+# Surface app-side errors, which sway captures on its stdout.
+grep -iE "error|warn|panic|segfault" "$RUNDIR/sway.log" | grep -v "portal" | head -20 || true