feat(layer-nerdctl): nested-pod posture + its R10 bed (Phase 4, v2)

.gitignore

@@ -4,3 +4,6 @@
 # Editor / OS
 .DS_Store
 *.swp
+.check/
+.containerignore
+.dockerignore

box/check-nerdctl-app/charly.yml

@@ -0,0 +1,18 @@
+# check-nerdctl-app — the disposable bed image for the layer-nerdctl R10: a Fedora
+# base composing the nerdctl stack candy, so the nested-pod posture (cap_add ALL,
+# /dev/fuse, /dev/net/tun, unmask /proc/*) is emitted onto a real pod.
+check-nerdctl-app:
+    candy:
+        version: 2026.266.1730
+        description: |
+            Disposable bed image for the layer-nerdctl R10 — a Fedora base composing
+            layer-nerdctl, so the nested-pod posture is emitted and can be asserted.
+        base: quay.io/fedora/fedora:43
+        candy:
+            - layer-nerdctl
+        plan:
+            - check: nerdctl is installed by the composed layer-nerdctl candy
+              command: command -v nerdctl
+              exit_status: 0
+            - check: the nerdctl.toml written by the candy is present
+              file: /etc/nerdctl/nerdctl.toml

candy/layer-nerdctl/charly.yml

@@ -22,6 +22,19 @@ layer-nerdctl:
             info, the rootless probe, and the fuse-overlayfs snapshotter.
         var:
             NERDCTL_FULL_VERSION: "2.1.2"
+        security:
+            # The nested-pod posture for the OUTER pod (userns-scoped): the inner
+            # rootless engine needs the full capability set WITHIN its user namespace,
+            # /dev/fuse for the fuse-overlayfs snapshotter, /dev/net/tun for the CNI
+            # bridge, and the /proc/* unmask so the inner runtime can re-mount /proc.
+            # These are userns-scoped — the inner engine's userns-root is NOT host root.
+            cap_add:
+                - ALL
+            devices:
+                - /dev/fuse
+                - /dev/net/tun
+            security_opt:
+                - unmask=/proc/*
         distro:
             arch:
                 package:
@@ -107,3 +120,46 @@ layer-nerdctl:
               exit_status: 0
             - check: the charly CNI network conflist is present
               file: /etc/cni/net.d/charly.conflist
+
+# ---- HARNESS SKILL ----
+layer-nerdctl-skill:
+    skill:
+        name: layer-nerdctl
+        family: distros
+        owner: layer-nerdctl
+        description: "nerdctl (containerd CLI) + the rootless containerd/buildkit/CNI stack — native Arch/Alpine packages, the pinned sha256 nerdctl-full tarball on Fedora/Debian/Ubuntu, nerdctl.toml + the charly CNI conflist + the nested-pod posture. Use when working with the nerdctl engine, the layer-nerdctl candy, or engine: nerdctl deploys."
+        content: |
+            # layer-nerdctl
+
+            nerdctl (the containerd CLI) plus the rootless containerd + buildkit + CNI
+            stack, as ONE candy. Composed by a box that wants the opt-in `engine: nerdctl`
+            engine.
+
+            ## What it installs
+
+            - **Native packages** on Arch/CachyOS/Omarchy and Alpine: `nerdctl`,
+              `cni-plugins`, `rootlesskit`, `slirp4netns`, `buildkit`, `fuse-overlayfs`,
+              `crun`, `iptables-nft` (arch) / `iptables` (alpine).
+            - **Non-native distros** (Fedora/Debian/Ubuntu) install the pinned,
+              sha256-verified `nerdctl-full` tarball (step `nerdctl-full-tarball`), which
+              supplies nerdctl + containerd + `containerd-fuse-overlayfs-grpc` + CNI +
+              buildkit + rootlesskit in one archive.
+            - `/etc/nerdctl/nerdctl.toml` (charly namespace + rootless buildkit host) and
+              the `charly` CNI network conflist at `/etc/cni/net.d/charly.conflist`.
+            - The **nested-pod posture**: userns-scoped `cap_add: ALL`, `/dev/fuse`,
+              `/dev/net/tun`, `unmask=/proc/*`.
+
+            ## Engine capability
+
+            The `engine: nerdctl` word is served by `opencharly/plugin-nerdctl`
+            (out-of-process) or compiled in; every engine op delegates to
+            `container.InvokeEngineOp("nerdctl", …)` in the spec module. nerdctl has no
+            `--keep-id`, so the start-plan runs the workload as `--user 0:0` (uid 0 == the
+            host user under rootless), guaranteeing uid-identical sharing; a non-zero uid
+            needs the subuid caveat.
+
+            ## Use when
+
+            Working with the nerdctl engine, the layer-nerdctl candy, or an
+            `engine: nerdctl` deploy.
+

charly.yml

@@ -26,3 +26,31 @@ defaults:
   platform:
     - linux/amd64
   build: [rpm]
+
+# check-nerdctl-nested — the R10 bed for the layer-nerdctl nested-pod posture: a
+# disposable pod composing the nerdctl stack, asserting the emitted devices +
+# capability posture on a FRESH deploy. Fails without the security: block.
+check-nerdctl-nested:
+    pod:
+        image: check-nerdctl-app
+        disposable: true
+        lifecycle: test
+        description: |
+            The nested-pod R10 bed: deploy a disposable pod composing layer-nerdctl and
+            assert the outer-pod posture the inner rootless engine needs — /dev/fuse,
+            /dev/net/tun, and the userns-scoped full capability set (cap_add: ALL).
+        plan:
+            - check: the fuse device is present for the fuse-overlayfs snapshotter
+              file: /dev/fuse
+            - check: the tun device is present for the CNI bridge
+              file: /dev/net/tun
+            - check: the exact full capability mask is retained (cap_add ALL)
+              command: grep -qE '^CapEff:\s+000001ffffffffff$' /proc/self/status
+              exit_status: 0
+            - check: the pod is userns-scoped (uid 0 inside maps to the host user, not host root)
+              command: grep -qE '^\s+0\s+1000\s+1\s*$' /proc/self/uid_map
+              exit_status: 0
+            - check: nerdctl is on PATH in the nested pod
+              command: command -v nerdctl
+              exit_status: 0
+