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.Tests/Fakes/FakePage.cs

@@ -3,9 +3,13 @@ using AvParser.UI.ViewModels;
 namespace AvParser.UI.Tests.Fakes;
 
 /// <summary>A navigation destination with no behaviour, for exercising the shell and the stack.</summary>
-internal sealed class FakePage(string title, string iconKey = "IconHome") : PageViewModel
+/// <remarks>
+/// Takes a real resource key rather than a literal caption, so the tests exercise the same
+/// lookup the app does instead of a special case that would hide a broken key.
+/// </remarks>
+internal sealed class FakePage(string titleKey, string iconKey = "IconHome") : PageViewModel
 {
-    public override string Title { get; } = title;
+    public override string TitleKey { get; } = titleKey;
 
     public override string IconKey { get; } = iconKey;
 }
@@ -13,7 +17,7 @@ internal sealed class FakePage(string title, string iconKey = "IconHome") : Page
 /// <summary>A second page type, so <c>NavigateTo&lt;TPage&gt;()</c> has something to discriminate on.</summary>
 internal sealed class OtherFakePage : PageViewModel
 {
-    public override string Title => "Other";
+    public override string TitleKey => "Page.About";
 
     public override string IconKey => "IconInfo";
 }