Add layout dialog helper.

build.zig

@@ -134,6 +134,22 @@ pub fn build(b: *std.Build) void {
     const test_step = b.step("test", "Run the tests");
     test_step.dependOn(&b.addRunArtifact(tests).step);
 
+    // The directory field's rules are their own root, for the same reason as
+    // the layouts file: which directory a half-typed path names, which entries
+    // in it are offered and how far they agree is text and filesystem, and the
+    // popover above it is the part that wants a display. It reaches GLib and
+    // nothing else, so it tests without one.
+    const paths_tests = b.addTest(.{
+        .root_module = b.createModule(.{
+            .root_source_file = b.path("src/paths.zig"),
+            .target = target,
+            .optimize = optimize,
+            .link_libc = true,
+        }),
+    });
+    paths_tests.root_module.addImport("glib", gobject.module("glib2"));
+    test_step.dependOn(&b.addRunArtifact(paths_tests).step);
+
     // The palette is the third root: a table of colours, the rules for
     // resolving one, and the CSS it is written out as. It reaches libghostty-vt
     // for the terminal's own default palette and nothing else, so it tests