Per-system bundles, and Android's start problem closed by narrowing it

Two gaps.

The bundle's contents are per system even though its mechanism is not, so there
are now three: substrate-arch.lock, substrate-alpine.lock and
substrate-android.lock. All three are embedded and a host reads only the one it
was built for. Arch and Alpine remain placeholders -- the closure for a one-node
mesh is still research 011/012's open question, and inventing it here would be
worse than an honest placeholder.

Android's is not a placeholder. It says a partial host cannot raise a mesh and
why: every step of a bootstrap is a package, a container, or an action against
one, and those are exactly the shapes it refuses. So a partial host can JOIN a
mesh and cannot BE the first node. That belongs where somebody looking for the
android bundle will find it.

Also separated two things that were being conflated: "this system has no
bundle" and "this system was never built". Loading a bundle for debian is not
ErrEmpty, and the test asserts they differ.

0062 -- a host may be episodic. There is no way to keep a process running on an
ordinary Android device: init needs root, a foreground service can be killed
for memory. The answer is not to fight that. It is that being killed IS
disconnection, which ADR 0036 already made an ordinary situation -- and
everything the design does for a laptop that closes is what an episodic host
needs, at a shorter period. An authoritative local store, reconcile on start,
last-heard-from reported without an alarm.

So the gap closes by requiring less rather than building something. No keep-
alive, no Android daemon, no fighting the platform's process management.

Two consequences recorded rather than glossed. Last-heard-from is a much weaker
signal on an episodic host, so a healthy phone reads as a dead server unless
the reader knows which kind it is looking at. And a declaration may take a long
time to land, which makes 0058's separation of outstanding from failed
load-bearing rather than tidy.

Left open deliberately: how an episodic host is actually started, and -- first --
what an Android node is for. Building the start mechanism before deciding that
would be building it for nobody.

cmd/mesh-host/main.go

@@ -176,23 +176,23 @@ func run(ctx context.Context, command string, opts options) error {
 		// comes from the bundle the host carries. There is no link yet, so this is currently
 		// the only source — which is a stage, not a design, and saying so beats implying the
 		// other source exists.
-		d, err := bundle.Load()
+		d, err := bundle.Load(builtFor)
 		if err != nil {
 			return err
 		}
 		return runApply(ctx, opts, d, "the carried bundle")
 
 	case "bundle":
-		if bundle.IsEmpty() {
+		if bundle.IsEmpty(builtFor) {
 			fmt.Println("this host carries no bundle")
 			return nil
 		}
-		_, err := bundle.Load()
+		_, err := bundle.Load(builtFor)
 		if err != nil {
 			// Asked before it matters, rather than discovered on a first node.
 			return fmt.Errorf("this host carries a bundle it cannot itself read: %w", err)
 		}
-		os.Stdout.Write(bundle.Raw())
+		os.Stdout.Write(bundle.Raw(builtFor))
 		return nil
 
 	case "owned":