Various changes.

flake.nix

@@ -5,6 +5,11 @@
     # Ghostty tracks nixpkgs-unstable to get GTK 4.20 / GNOME 49. We follow
     # suit so that our zig-gobject bindings (shared with Ghostty) match the
     # GObject introspection data of the GTK we link against.
+    #
+    # Follow the channel, not necessarily Ghostty's exact lock: the binary
+    # runs on the host's dynamic loader and libc, so a nixpkgs whose glibc is
+    # newer than the host's builds fine and then fails to start with
+    # "GLIBC_2.43 not found". What has to match is GTK's version, not the rev.
     nixpkgs.url = "https://channels.nixos.org/nixpkgs-unstable/nixexprs.tar.xz";
 
     flake-compat = {
@@ -17,7 +22,7 @@
       flake = false;
     };
 
-    # Zig 0.16.0 is not in nixpkgs yet; libghostty-vt requires it.
+    # Zig 0.17.0 is not in nixpkgs yet; libghostty-vt requires it.
     zig = {
       url = "github:mitchellh/zig-overlay";
       inputs = {
@@ -45,7 +50,7 @@
         name = "playpen";
 
         nativeBuildInputs = [
-          zig.packages.${system}."0.16.0"
+          zig.packages.${system}."0.17.0"
           pkgs.pkg-config
           pkgs.gdb