Remove the demo text-parsing domain

The scaffolding domain existed to prove the shell end to end before there was
anything real to put in it. There is now, so it goes - as CLAUDE.md promised it
would.

Gone: the two sample parsers, ITextParser, ParsedRecord, the parser catalog,
ParseViewModel and ParseView, their tests, and the settings key that remembered
which parser was last used. ParseError.LineNumber becomes Index, since for a
listing "line 42" was simply untrue, and the error keys move from Parse.Error.*
to Collect.Error.* now that parsing is not a concept here.

Kept: IParser<,>, ParseOutcome, ParseProgress and ParseError. The streaming
contract was always the general part - it was only ever the text-shaped closure
of it that was scaffolding.

Rendering the dashboard caught two keys that were referenced but never added
during the rename: the XAML was repointed and the resources were not. The parity
test could not see it, because it compares the two files against each other and
a key absent from both is consistent. That gap now has its own test, which reads
every {l:Loc} in the XAML and checks it resolves - a screenshot is too late and
too manual a way to find a missing string.

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

tests/AvParser.UI.Tests/LocalizationCoverageTests.cs

@@ -0,0 +1,65 @@
+using System.Reflection;
+using System.Resources;
+using System.Text.RegularExpressions;
+using AvParser.UI.Localization;
+
+namespace AvParser.UI.Tests;
+
+/// <summary>
+/// Every key the XAML asks for must actually exist.
+/// </summary>
+/// <remarks>
+/// The parity test compares the two resource files against each other, so a key missing from
+/// <b>both</b> passes it happily and then renders as <c>!Some.Key!</c> in the running app. That
+/// happened during a rename: the XAML was repointed at new key names and the keys themselves were
+/// never added. Only a screenshot caught it, which is too late and too manual.
+/// </remarks>
+public sealed partial class LocalizationCoverageTests
+{
+    [GeneratedRegex(@"\{l:Loc\s+([A-Za-z0-9_.\-]+)\s*\}", RegexOptions.Compiled)]
+    private static partial Regex LocMarkup();
+
+    private static readonly ResourceManager Resources = new(
+        "AvParser.UI.Localization.Strings",
+        typeof(Localizer).GetTypeInfo().Assembly
+    );
+
+    /// <summary>Walks up to the repository root, which the test binary sits several levels below.</summary>
+    private static DirectoryInfo RepositoryRoot()
+    {
+        var directory = new DirectoryInfo(AppContext.BaseDirectory);
+
+        while (directory is not null && !File.Exists(Path.Combine(directory.FullName, "AvParser.slnx")))
+        {
+            directory = directory.Parent;
+        }
+
+        return directory ?? throw new InvalidOperationException("Could not find the repository root.");
+    }
+
+    [Fact]
+    public void Every_key_used_in_xaml_exists()
+    {
+        var views = Path.Combine(RepositoryRoot().FullName, "src", "AvParser.UI");
+        var files = Directory.GetFiles(views, "*.axaml", SearchOption.AllDirectories);
+
+        files.ShouldNotBeEmpty();
+
+        var missing = new SortedSet<string>(StringComparer.Ordinal);
+
+        foreach (var file in files)
+        {
+            foreach (Match match in LocMarkup().Matches(File.ReadAllText(file)))
+            {
+                var key = match.Groups[1].Value;
+
+                if (Resources.GetString(key, Localizer.Instance.Culture) is null)
+                {
+                    missing.Add($"{key} ({Path.GetFileName(file)})");
+                }
+            }
+        }
+
+        missing.ShouldBeEmpty();
+    }
+}