Add test coverage for cmd/vintner and internal/i18n

Both were at 0% coverage. Focused on what's safely testable without
touching the network or filesystem: flag validation (--architecture/
--host-arch, the same guard added in the stability pass), subcommand
dispatch and aliases, help/usage error paths, and - for i18n - full
language-detection table coverage plus a completeness check that
every catalog key has both an EN and RU entry (an English-only or
Russian-only entry would silently degrade rather than fail loudly,
so this is worth locking in). cmd/vintner: 0% -> 28.8%, i18n: 0% ->
94.1%.

cmd/vintner/install_test.go

@@ -0,0 +1,17 @@
+package main
+
+import "testing"
+
+func TestRunInstallRejectsExtraArgs(t *testing.T) {
+	if code := runInstall([]string{"one", "two"}); code != 1 {
+		t.Errorf("runInstall with two args = %d, want 1", code)
+	}
+}
+
+func TestRunInstallHelp(t *testing.T) {
+	for _, flag := range []string{"-h", "--help"} {
+		if code := runInstall([]string{flag}); code != 1 {
+			t.Errorf("runInstall([%q]) = %d, want 1", flag, code)
+		}
+	}
+}