Live-validation fixes for the dark containerd engine (#329), on top of v0.7.3. Behind SB_CONTAINER_ENGINE=containerd; docker default untouched.

• CNI_ARGS IgnoreUnknown=true: plugins rejected every ADD with code 999: unknown args [K8S_POD_*] (types.LoadArgs errors on undefined keys). Now prefixed as Kubernetes does. Verified: bridge allocates 10.88.0.2/16.
• bootstrap.sh.tftpl $(uname -m) escaping: $$(uname -m) rendered literally in the Terraform template → arch match failed → exit 1 aborted the bootstrap before the sandboxd restart, leaving containerd nodes with no CNI plugins and the daemon on docker. Fixed.

With v0.7.3's real-netns + stdout-error-surfacing fixes, a freshly provisioned containerd node now boots the engine, installs CNI plugins, and creates sandboxes on aerolvm0. Still dark; §8 gates remain the flip bar.

What's Changed

• fix(containerd): CNI_ARGS IgnoreUnknown + bootstrap uname escaping (live-validated) by @sumansaurabh in #329

Full Changelog: v0.7.3...v0.7.4