Localise the UI into Russian and English, switchable without a restart

Strings move into Localization/Strings.resx plus a Russian satellite. XAML uses
a {l:Loc Key} markup extension that yields a binding through the localizer's
indexer rather than a resolved string, so changing the language raises
PropertyChanged for the indexer and every caption in the app re-reads at once.
Resolving strings once at load would have been simpler and would have needed an
app restart to take effect.

Three places where this is more than a string swap:

Russian has three plural forms, so counted messages are assembled from .One /
.Few / .Many keys via Localizer.Plural instead of "{0} records" with an English
plural glued on. "1 запись", "3 записи", "7 записей".

Enum captions in pickers go through LocalizedOption<T>. A value converter would
resolve the caption once and never notice a language change; the wrapper keeps
identity on the enum value so the selection survives, while the label follows
the localizer. It needed a non-generic base for DataTemplate x:DataType, since
Avalonia 12 compiles bindings by default and cannot infer one for an open
generic.

Text originating in the domain would otherwise have stayed English under a
Russian UI — the screenshots showed exactly that. AvParser.Core still knows
nothing about languages: ParseError now carries a Code and Arguments, and the UI
translates Parse.Error.{Code} with a fallback to the English message. Parser
names work the same way (Parser.{id}.Name falling back to DisplayName), which
keeps "add a parser = one registration line" true — an untranslated parser shows
its own name rather than a missing-key marker.

Both .resx files are generated from one table so a key cannot exist in one and
be missing from the other, and the tests assert that, plus no blank translations
and identical {0} placeholder sets — a translation that drops a placeholder
throws at runtime rather than merely reading oddly. A headless test switches
language on a live shell and asserts the rendered text changes without the tree
being rebuilt.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

tests/AvParser.UI.HeadlessTests/ShellViewTests.cs

@@ -5,6 +5,7 @@ using Avalonia.Styling;
 using Avalonia.Threading;
 using Avalonia.VisualTree;
 using AvParser.Core.Settings;
+using AvParser.UI.Localization;
 using AvParser.UI.Navigation;
 using AvParser.UI.Responsive;
 using AvParser.UI.Services;
@@ -27,7 +28,10 @@ public class ShellViewTests
     /// </remarks>
     private static (ShellView View, ShellViewModel ViewModel, Window Window) ShowShell(double width)
     {
-        var navigation = new NavigationService([new FakePage("First"), new FakePage("Second", "IconDocument")]);
+        var navigation = new NavigationService([
+            new FakePage("Page.Dashboard"),
+            new FakePage("Page.Parse", "IconDocument"),
+        ]);
 
         // ImmediateSequencer, not the real main-thread one: derived properties then settle within
         // the same layout pass, so a single RunJobs() is enough for the bindings to catch up.
@@ -108,7 +112,7 @@ public class ShellViewTests
 
         var host = view.GetVisualDescendants().OfType<TransitioningContentControl>().Single();
         host.Content.ShouldBeSameAs(second);
-        viewModel.Title.ShouldBe("Second");
+        viewModel.Title.ShouldBe(Localizer.Instance["Page.Parse"]);
     }
 
     /// <summary>