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.

b.sh

@@ -0,0 +1,7 @@
+#!/usr/bin/env bash
+# Build helper: zig build emits an enormous "failed command" line on error,
+# which drowns out the actual diagnostics. Strip it.
+cd "$(dirname "$0")"
+nix develop --command zig build "$@" 2>&1 \
+  | grep -v "^warning: Git tree" \
+  | sed '/^failed command:/,$d'