Rename mesh-control -> mesh-controller, substrate -> foundation

One name per thing, per the HQ glossary: the module/container/image/binary/repo
becomes mesh-controller, the seat the-controller, and the store+broker pair the
foundation (embedded base bundles, default template and example lock renamed with
their go:embed directives). No behaviour change — a pure vocabulary rename.

Claude-Session: https://claude.ai/code/session_01D6qtiYU3P9jk3pnAXyAFyx

internal/bundle/bundle_test.go

@@ -13,7 +13,7 @@ func TestADefaultBuildCarriesNothingAndSaysSo(t *testing.T) {
 	// success would look exactly like a host that raised a first node — and the difference
 	// would surface as a mesh that never came up, with nothing to point at.
 	if !IsEmpty("arch") {
-		t.Fatal("the default build claims to carry a substrate")
+		t.Fatal("the default build claims to carry a foundation")
 	}
 	_, err := Load("arch")
 	if !errors.Is(err, ErrEmpty) {