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)
+	}
+}

internal/i18n/i18n_test.go

@@ -0,0 +1,93 @@
+package i18n
+
+import (
+	"strings"
+	"testing"
+)
+
+func TestDetect(t *testing.T) {
+	envKeys := []string{"VINTNER_LANG", "LC_ALL", "LC_MESSAGES", "LANG"}
+	clear := func() {
+		for _, k := range envKeys {
+			t.Setenv(k, "")
+			// t.Setenv("", "") leaves the var set-but-empty, which detect()
+			// already treats as "unset" (its loop skips v == "") - matches
+			// how a genuinely-unset env var behaves for this function.
+		}
+	}
+
+	for _, tc := range []struct {
+		name string
+		env  map[string]string
+		want Lang
+	}{
+		{"nothing set defaults to English", nil, EN},
+		{"VINTNER_LANG=ru", map[string]string{"VINTNER_LANG": "ru"}, RU},
+		{"VINTNER_LANG=en", map[string]string{"VINTNER_LANG": "en"}, EN},
+		{"VINTNER_LANG wins over a Russian LANG", map[string]string{"VINTNER_LANG": "en", "LANG": "ru_RU.UTF-8"}, EN},
+		{"LC_ALL wins over LANG", map[string]string{"LC_ALL": "ru_RU.UTF-8", "LANG": "en_US.UTF-8"}, RU},
+		{"LANG=ru_RU.UTF-8 alone", map[string]string{"LANG": "ru_RU.UTF-8"}, RU},
+		{"LANG=en_US.UTF-8 alone", map[string]string{"LANG": "en_US.UTF-8"}, EN},
+		{"unrelated locale defaults to English", map[string]string{"LANG": "de_DE.UTF-8"}, EN},
+		{"case-insensitive RU prefix", map[string]string{"VINTNER_LANG": "RU"}, RU},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			clear()
+			for k, v := range tc.env {
+				t.Setenv(k, v)
+			}
+			if got := detect(); got != tc.want {
+				t.Errorf("detect() = %q, want %q", got, tc.want)
+			}
+		})
+	}
+}
+
+// TestCatalogCompleteness guards against adding an EN string without its RU
+// counterpart (or vice versa) - a silent gap here degrades to showing the
+// wrong language's text via T()'s EN-fallback rather than failing loudly.
+func TestCatalogCompleteness(t *testing.T) {
+	for key, entry := range catalog {
+		en, hasEN := entry[EN]
+		if !hasEN || strings.TrimSpace(en) == "" {
+			t.Errorf("catalog[%q] has no (non-empty) English translation", key)
+		}
+		ru, hasRU := entry[RU]
+		if !hasRU || strings.TrimSpace(ru) == "" {
+			t.Errorf("catalog[%q] has no (non-empty) Russian translation", key)
+		}
+	}
+}
+
+func TestTMissingKeyReturnsKeyItself(t *testing.T) {
+	got := T("no.such.key")
+	if got != "no.such.key" {
+		t.Errorf("T(unknown key) = %q, want the key itself", got)
+	}
+}
+
+func TestTFallsBackToEnglish(t *testing.T) {
+	const testKey = "test.fallback.only.en"
+	catalog[testKey] = map[Lang]string{EN: "hello %s"}
+	defer delete(catalog, testKey)
+
+	saved := current
+	current = RU
+	defer func() { current = saved }()
+
+	if got := T(testKey, "world"); got != "hello world" {
+		t.Errorf("T(%q) with no RU entry = %q, want %q", testKey, got, "hello world")
+	}
+}
+
+func TestTFormatsArgs(t *testing.T) {
+	saved := current
+	current = EN
+	defer func() { current = saved }()
+
+	got := T("download.wdk_installed", "x64", "10.0.26100.1", "/dest")
+	want := "Installed WDK (x64) 10.0.26100.1 at /dest\n"
+	if got != want {
+		t.Errorf("T(download.wdk_installed, ...) = %q, want %q", got, want)
+	}
+}