feat(i18n): add Localizer with fallback chain and change event

tests/ClaudeDo.Localization.Tests/LocalizerTests.cs

@@ -0,0 +1,58 @@
+using ClaudeDo.Localization;
+
+namespace ClaudeDo.Localization.Tests;
+
+public class LocalizerTests
+{
+    private static LocaleStore TwoLangStore()
+    {
+        var dir = Path.Combine(Path.GetTempPath(), "loc_" + Guid.NewGuid().ToString("N"));
+        Directory.CreateDirectory(dir);
+        File.WriteAllText(Path.Combine(dir, "en.json"),
+            """{ "metadata": { "code": "en", "name": "English" }, "settings": { "save": "Save" }, "msg": { "count": "{0} items" } }""");
+        File.WriteAllText(Path.Combine(dir, "de.json"),
+            """{ "metadata": { "code": "de", "name": "Deutsch" }, "settings": { "save": "Speichern" } }""");
+        return LocaleStore.Load(dir);
+    }
+
+    [Fact]
+    public void Indexer_returns_active_language_value()
+    {
+        var loc = new Localizer(TwoLangStore(), "de", fallbackCode: "en");
+        Assert.Equal("Speichern", loc["settings.save"]);
+    }
+
+    [Fact]
+    public void Indexer_falls_back_to_english_then_key()
+    {
+        var loc = new Localizer(TwoLangStore(), "de", fallbackCode: "en");
+        Assert.Equal("{0} items", loc["msg.count"]);
+        Assert.Equal("nope.missing", loc["nope.missing"]);
+    }
+
+    [Fact]
+    public void Get_formats_arguments()
+    {
+        var loc = new Localizer(TwoLangStore(), "en", fallbackCode: "en");
+        Assert.Equal("3 items", loc.Get("msg.count", 3));
+    }
+
+    [Fact]
+    public void SetLanguage_raises_change_and_switches_values()
+    {
+        var loc = new Localizer(TwoLangStore(), "en", fallbackCode: "en");
+        var raised = 0;
+        loc.LanguageChanged += (_, _) => raised++;
+        loc.SetLanguage("de");
+        Assert.Equal("de", loc.CurrentCode);
+        Assert.Equal("Speichern", loc["settings.save"]);
+        Assert.Equal(1, raised);
+    }
+
+    [Fact]
+    public void AvailableLanguages_lists_code_and_name()
+    {
+        var loc = new Localizer(TwoLangStore(), "en", fallbackCode: "en");
+        Assert.Contains(loc.AvailableLanguages, l => l.Code == "de" && l.Name == "Deutsch");
+    }
+}