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/style.css

@@ -0,0 +1,66 @@
+/* Zen-style vertical tabs: a dark sidebar column with the terminal inset
+   to its right. */
+
+.vtabs-window {
+  background-color: #0f0d14;
+}
+
+.vtabs-sidebar {
+  background-color: #1b1823;
+  border-right: 1px solid #2a2536;
+}
+
+.vtabs-sidebar headerbar {
+  background: none;
+  box-shadow: none;
+  min-height: 38px;
+}
+
+.vtabs-list {
+  background: none;
+  padding: 4px 6px;
+}
+
+.vtabs-list > row {
+  border-radius: 8px;
+  margin: 1px 0;
+  padding: 5px 8px;
+  color: #b6afc7;
+  transition: background-color 120ms ease;
+}
+
+.vtabs-list > row:hover {
+  background-color: #262133;
+}
+
+.vtabs-list > row:selected {
+  background-color: #342c4a;
+  color: #f0ecf8;
+}
+
+.vtabs-list > row:selected image {
+  color: #b29df5;
+}
+
+/* Keep the close button unobtrusive until the row is hovered or current. */
+.vtabs-close {
+  opacity: 0;
+  min-width: 20px;
+  min-height: 20px;
+  padding: 0;
+}
+
+.vtabs-list > row:hover .vtabs-close,
+.vtabs-list > row:selected .vtabs-close {
+  opacity: 0.65;
+}
+
+.vtabs-close:hover {
+  opacity: 1;
+}
+
+.vtabs-content {
+  background-color: #16141c;
+  border-radius: 10px;
+  margin: 6px 6px 6px 0;
+}