feat: supabase stack seeds a requires on the substrate at create

A stack's substrate dependency now becomes a real encoded `requires` at
`castle program create` time (STACK_REQUIRES), so the relationship graph shows it.
This keeps `stack` uncoupled from the runtime model — the stack declares the edge
once at creation; the graph only ever reads the encoded `requires`, never the stack.

cli/tests/test_create.py

@@ -107,6 +107,9 @@ class TestCreateCommand:
         comp = config.programs["guestbook"]
         assert comp.stack == "supabase"
         assert comp.build is not None and comp.build.outputs == ["public"]
+        # The supabase stack seeds a `requires` on the substrate so the graph shows
+        # the dependency (stack stays uncoupled — it just declares the edge once).
+        assert [(r.kind, r.ref) for r in comp.requires] == [("deployment", "supabase")]
         dep = config.deployments["guestbook"]
         assert dep.manager == "caddy"
         assert dep.root == "public"