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/env_test.go

@@ -0,0 +1,23 @@
+package main
+
+import "testing"
+
+func TestRunEnvRequiresBin(t *testing.T) {
+	if code := runEnv(nil); code != 1 {
+		t.Errorf("runEnv(nil) = %d, want 1", code)
+	}
+}
+
+func TestRunEnvRejectsMissingBinDir(t *testing.T) {
+	if code := runEnv([]string{"--bin", "/nonexistent/path/for/vintner/tests"}); code != 1 {
+		t.Errorf("runEnv with a nonexistent --bin = %d, want 1", code)
+	}
+}
+
+func TestToUnixPathList(t *testing.T) {
+	got := toUnixPathList(`z:\vc\include;z:\kits\10\include`)
+	want := "/vc/include;/kits/10/include"
+	if got != want {
+		t.Errorf("toUnixPathList(...) = %q, want %q", got, want)
+	}
+}