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.

src/main.zig

@@ -0,0 +1,62 @@
+//! vtabs: a terminal with vertical tabs, built on libghostty-vt.
+//!
+//! libghostty-vt supplies the terminal emulator core (escape sequence
+//! parsing, screen and scrollback state, key/mouse encoding). Everything
+//! else — process management, rendering, and the GTK4 UI — lives here.
+
+const std = @import("std");
+const adw = @import("adw");
+const gdk = @import("gdk");
+const gio = @import("gio");
+const gtk = @import("gtk");
+
+const Window = @import("Window.zig");
+
+/// libghostty-vt logs unimplemented sequences at debug level, which is very
+/// chatty against a real shell. Keep the app's own warnings and errors.
+pub const std_options: std.Options = .{
+    .log_level = .info,
+};
+
+const css = @embedFile("style.css");
+
+var gpa: std.heap.DebugAllocator(.{}) = .init;
+
+pub fn main() u8 {
+    defer _ = gpa.deinit();
+
+    // Non-unique so every launch is its own process. The default GApplication
+    // behavior hands off to an already-running instance over D-Bus, which for
+    // a terminal means a second launch silently does nothing visible here and
+    // opens a window in whatever session owns the first one.
+    const app = adw.Application.new("dev.greyson.vtabs", .{ .non_unique = true });
+    defer app.unref();
+
+    _ = gio.Application.signals.activate.connect(app, ?*anyopaque, &onActivate, null, .{});
+
+    const status = gio.Application.run(app.as(gio.Application), 0, null);
+    return @intCast(status);
+}
+
+fn onActivate(app: *adw.Application, _: ?*anyopaque) callconv(.c) void {
+    loadCss();
+
+    const window = Window.create(gpa.allocator(), app) catch |err| {
+        std.log.err("failed to create window: {s}", .{@errorName(err)});
+        return;
+    };
+    window.present();
+}
+
+fn loadCss() void {
+    const display = gdk.Display.getDefault() orelse return;
+    const provider = gtk.CssProvider.new();
+    defer provider.unref();
+
+    provider.loadFromString(css);
+    gtk.StyleContext.addProviderForDisplay(
+        display,
+        provider.as(gtk.StyleProvider),
+        gtk.STYLE_PROVIDER_PRIORITY_APPLICATION,
+    );
+}