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.lock

@@ -59,11 +59,11 @@
         ]
       },
       "locked": {
-        "lastModified": 1779533061,
-        "narHash": "sha256-orWNYXtYURhEj3X4+xGMAhaEcKRvwXqTtJ8x2jV/M+Q=",
+        "lastModified": 1780368078,
+        "narHash": "sha256-tLzA5XveUF4PfuKNz3KuhmVhuME3PX5zvtFa17hhQPU=",
         "ref": "refs/heads/main",
-        "rev": "b818e345ec4470e4b3e335bd2f864183c512116d",
-        "revCount": 13,
+        "rev": "1626405d46ff3595b91c9e2d3ed9399f67c18b83",
+        "revCount": 15,
         "type": "git",
         "url": "https://git.oleks.space/oleks/fleet-pins"
       },

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;
       }
     );
 }

lib/image-layers.nix

@@ -0,0 +1,37 @@
+# Pkgs-independent nix2container layer helpers, shared across the parity image
+# repos (ii-agent, ii-researcher, temporal-based-ci, mempalace image,
+# ComfyUI, …) so the reproducible=false rationale lives in exactly one place.
+{
+  # foldImageLayers buildLayer layers
+  #
+  #   buildLayer : nix2container's buildLayer function. Pass whichever attr the
+  #                consumer's nix2container input exposes —
+  #                `n2c.nix2container.buildLayer` or `n2c.buildLayer`.
+  #   layers     : list of buildLayer component attrsets (deps / copyToRoot /
+  #                perms / …) in base→top order.
+  #
+  # Each layer is built referencing all prior layers, with reproducible = false.
+  # That is a DELIBERATE choice: it materialises each layer tar into the store so
+  # the image streams verbatim from any host (remote-builder + binary-cache safe)
+  # and avoids the cross-host "Digest did not match" that non-reproducible layer
+  # deps (fenix rust, libllvm, …) otherwise trigger via nix2container's lazy tar
+  # regeneration. Parity is asserted at the published digest, not byte-identical
+  # tars — mkNix2ContainerPublish enforces that contract downstream.
+  foldImageLayers =
+    buildLayer: layers:
+    let
+      mergeToLayer =
+        priorLayers: component:
+        priorLayers
+        ++ [
+          (buildLayer (
+            component
+            // {
+              layers = priorLayers;
+              reproducible = false;
+            }
+          ))
+        ];
+    in
+    builtins.foldl' mergeToLayer [ ] layers;
+}