Add ability to restore previous layout.

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 session snapshot is its own root for the same reason, and is the same
+    // kind of thing: a file the app writes and reads back, whose shape is worth
+    // being sure about. It imports Layouts.zig for the node grammar the two
+    // share, and needs libc for the one call GLib does not expose — see
+    // `Snapshot.remove`.
+    const snapshot_tests = b.addTest(.{
+        .root_module = b.createModule(.{
+            .root_source_file = b.path("src/Snapshot.zig"),
+            .target = target,
+            .optimize = optimize,
+            .link_libc = true,
+        }),
+    });
+    snapshot_tests.root_module.addImport("glib", gobject.module("glib2"));
+    test_step.dependOn(&b.addRunArtifact(snapshot_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