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.

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;
+}