feat(lib): add foldImageLayers helper + formatter; refresh fleet pin

Lift the nix2container reproducible=false layer-chain helper (duplicated
verbatim across ii-agent, ii-researcher, temporal-based-ci, mempalace image,
ComfyUI) into parity.lib.foldImageLayers so the rationale lives in one place.
Add nixfmt-rfc-style formatter and a foldImageLayers contract probe to the
smoke check. Bump fleet-pins input to current HEAD.

flake.nix

@@ -28,9 +28,13 @@
           wrap =
             name: pkgs: args:
             (builders pkgs).${name} args;
+          imageLayers = import ./lib/image-layers.nix;
         in
         {
           mkParityBuilders = builders;
+          # Pkgs-independent nix2container layer-chain helper (lib/image-layers.nix):
+          #   parity.lib.foldImageLayers buildLayer [ { deps = …; } … ]
+          inherit (imageLayers) foldImageLayers;
           mkPyPiWheelPublish = wrap "mkPyPiWheelPublish";
           mkPyPiWheelPublishMulti = wrap "mkPyPiWheelPublishMulti";
           mkS390xNpmPublish = wrap "mkS390xNpmPublish";
@@ -60,8 +64,15 @@
           text = ''exec bash ${./ci/pipeline-doctor.sh} "$@"'';
         };
 
-        # Smoke check: instantiate every builder with stub args so the apps eval.
+        # Smoke check: instantiate a builder with stub args so the apps eval, and
+        # exercise foldImageLayers with a stub buildLayer so its contract (each
+        # layer gets reproducible=false + the prior layers) can't silently break.
         builders = import ./lib/builders.nix { inherit pkgs; };
+        imageLayers = import ./lib/image-layers.nix;
+        layerProbe = imageLayers.foldImageLayers (c: c) [
+          { deps = [ pkgs.hello ]; }
+          { deps = [ pkgs.coreutils ]; }
+        ];
         smoke = pkgs.runCommand "parity-lib-smoke" { } ''
           : "${
             builtins.toString (
@@ -74,6 +85,9 @@
               )
             )
           }"
+          ${lib.optionalString (
+            builtins.length layerProbe != 2 || (builtins.elemAt layerProbe 1).reproducible
+          ) ''echo "foldImageLayers contract broken" >&2; exit 1''}
           touch $out
         '';
       in
@@ -90,6 +104,8 @@
         };
 
         checks.smoke = smoke;
+
+        formatter = pkgs.nixfmt-rfc-style;
       }
     );
 }