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.

.gitignore

@@ -0,0 +1,6 @@
+.zig-cache/
+zig-out/
+# Zig 0.16 unpacks fetched dependencies here; they're reproducible from
+# build.zig.zon and must not be committed.
+zig-pkg/
+result