fix(overlay): make meshtastic conditional for stable nixpkgs (25.11)

meshtastic package doesn't exist in nixpkgs 25.11 stable.
Use optionalAttrs to skip the override when not available,
allowing the flake to build against both stable and unstable.

overlays/reticulum.nix

@@ -1,4 +1,5 @@
 final: prev: let
+  inherit (final.lib) optionalAttrs;
   python3 = final.python3;
   pyPkgs = python3.pkgs;
 in {
@@ -78,8 +79,9 @@ in {
       platforms = platforms.linux;
     };
   };
-
-  # meshtastic requires tabulate < 0.10.0 but nixpkgs has 0.10.0
+}
+# meshtastic may not be in all nixpkgs versions (not in 25.11 stable)
+// optionalAttrs (builtins.hasAttr "meshtastic" prev) {
   meshtastic = prev.meshtastic.overrideAttrs (old: {
     dontCheckRuntimeDeps = true;
   });