A user, bytes, and an archive — because most of what people install is

not a service

A shell, a terminal, a chat client, a desktop are a package plus
configuration in somebody's home. A mesh with no notion of a user can own
/etc and nothing anybody looks at, which is most of the reason to manage
a machine at all.

Three shapes, and the vocabulary test asserts the count precisely because
widening it widens what a compromised control plane can express:

  user     a login, its shell and its groups
  archive  a set of files, fetched by digest and unpacked
  (file)   gains `bytes` for what is not text, and `owner`

`user` also makes "zsh is my login shell" declared state. chsh is a
command, the link may not carry one, and a shell settable only by hand is
a shell the mesh cannot manage.

Groups are additive and never pruned — usermod without --append REPLACES
them, which would silently remove every group that makes a login able to
use the machine. A machine's own groups are not the mesh's to know about.

The archive is the one place this host reaches out on its own; everywhere
else it holds one outbound connection and fetches nothing. So it carries
the discipline the bootstrap already uses for images: pinned by digest,
and the digest checked before a single file is written.

Two decisions in the unpacker worth naming:

- an entry naming a path outside the archive is REFUSED, not sanitised.
  Rewriting it to land inside would put a file somewhere nobody asked for
  and report success. Found by the test: the first version quietly
  relocated it.
- symlinks and device nodes are refused rather than skipped, or an
  archive that needed one arrives silently incomplete.

A partial host does archives and refuses users: an archive needs a
filesystem and a way to fetch; a user needs a user database it is allowed
to write.

internal/system/android.go

@@ -80,3 +80,21 @@ func (a android) ServiceBoot(context.Context, Runner, string) (string, error) {
 func (a android) SetServiceBoot(context.Context, Runner, string, string) error {
 	return fmt.Errorf("%w: service", ErrUnsupported)
 }
+
+// Users are one of the shapes this host refuses.
+//
+// Android's user database belongs to the framework and is not something an ordinary app may
+// write. Refused with a reason rather than attempted, the same as package, service and container
+// above — and the profile says so, so the control plane never sends one.
+func (android) CreateUser(context.Context, Runner, string, string, string) error {
+	return fmt.Errorf("this host implements no users: Android's user database belongs to the " +
+		"framework and is not writable by an ordinary process")
+}
+
+func (android) SetUserShell(context.Context, Runner, string, string) error {
+	return fmt.Errorf("this host implements no users")
+}
+
+func (android) AddUserToGroup(context.Context, Runner, string, string) error {
+	return fmt.Errorf("this host implements no users")
+}