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

cmd/mesh-bootstrap/main_test.go

@@ -57,7 +57,7 @@ func TestAMistypedFlagIsRefusedNotIgnored(t *testing.T) {
 }
 
 func TestAnUnexpectedArgumentIsRefused(t *testing.T) {
-	if _, _, _, err := parseArgs([]string{"bootstrap", "substrate.lock"}); err == nil {
+	if _, _, _, err := parseArgs([]string{"bootstrap", "foundation.lock"}); err == nil {
 		t.Fatal("a stray argument was ignored rather than refused — the bundle is --bundle")
 	}
 }
@@ -141,7 +141,7 @@ func TestThePivotsDefaultsAreTheDocumentedOnes(t *testing.T) {
 	// be a checkout somebody else made, at whatever commit they left it on — and it decides which
 	// image the mesh's control plane is pinned to for ever after.
 	if opts.Catalogue != "" {
-		t.Errorf("--catalog defaults to %q; without one the installer stops at the substrate",
+		t.Errorf("--catalog defaults to %q; without one the installer stops at the foundation",
 			opts.Catalogue)
 	}
 	// The machine's own name, because that is what a person already calls it.