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/download_test.go

@@ -0,0 +1,40 @@
+package main
+
+import "testing"
+
+// TestRunDownloadRejectsInvalidArchFlags exercises the validation added
+// after the flags are parsed, which must reject typos before runDownload
+// gets anywhere near the network (FetchChannelManifest) - these tests would
+// hang/fail on network access if that ordering ever regressed.
+func TestRunDownloadRejectsInvalidArchFlags(t *testing.T) {
+	for _, tc := range []struct {
+		name string
+		args []string
+	}{
+		{"bad architecture", []string{"--architecture", "x866"}},
+		{"bad architecture, valid mixed with invalid", []string{"--architecture", "x64", "--architecture", "sparc"}},
+		{"bad host-arch", []string{"--host-arch", "sparc"}},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			if code := runDownload(tc.args); code != 2 {
+				t.Errorf("runDownload(%v) = %d, want 2", tc.args, code)
+			}
+		})
+	}
+}
+
+func TestValidArchitectureSets(t *testing.T) {
+	for _, a := range []string{"x86", "x64", "arm", "arm64", "host"} {
+		if !validArchitectures[a] {
+			t.Errorf("validArchitectures[%q] = false, want true", a)
+		}
+	}
+	for _, a := range []string{"x86", "x64", "arm64"} {
+		if !validHostArchs[a] {
+			t.Errorf("validHostArchs[%q] = false, want true", a)
+		}
+	}
+	if validHostArchs["arm"] {
+		t.Error(`validHostArchs["arm"] = true, want false (no 32-bit ARM host toolchain exists)`)
+	}
+}

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)
+	}
+}

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)
+		}
+	}
+}

cmd/vintner/main_test.go

@@ -0,0 +1,30 @@
+package main
+
+import "testing"
+
+// TestRunCLIDispatch covers the subset of runCLI's switch that has no side
+// effects (no filesystem/network touched) - the actual subcommand bodies
+// (download/install/env) get their own focused tests.
+func TestRunCLIDispatch(t *testing.T) {
+	for _, tc := range []struct {
+		name string
+		args []string
+		want int
+	}{
+		{"no args prints usage", nil, 1},
+		{"unknown subcommand", []string{"frobnicate"}, 1},
+		{"help long", []string{"--help"}, 0},
+		{"help short flag", []string{"-h"}, 0},
+		{"help word", []string{"help"}, 0},
+		{"help alias", []string{"h"}, 0},
+		{"version word", []string{"version"}, 0},
+		{"version alias", []string{"v"}, 0},
+		{"version flag", []string{"--version"}, 0},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			if got := runCLI(tc.args); got != tc.want {
+				t.Errorf("runCLI(%v) = %d, want %d", tc.args, got, tc.want)
+			}
+		})
+	}
+}

cmd/vintner/paths_test.go

@@ -0,0 +1,22 @@
+package main
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func TestDefaultToolchainDir(t *testing.T) {
+	home, err := os.UserHomeDir()
+	if err != nil {
+		t.Skipf("no home directory available: %v", err)
+	}
+	got, err := defaultToolchainDir()
+	if err != nil {
+		t.Fatalf("defaultToolchainDir() error: %v", err)
+	}
+	want := filepath.Join(home, ".vintner")
+	if got != want {
+		t.Errorf("defaultToolchainDir() = %q, want %q", got, want)
+	}
+}