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>

src/AvParser.Core/Parsing/ParseError.cs

@@ -1,9 +1,9 @@
 namespace AvParser.Core.Parsing;
 
 /// <summary>A recoverable problem with a single record. Parsing continues after one of these.</summary>
-/// <param name="LineNumber">1-based position of the offending record in the input.</param>
+/// <param name="Index">1-based position of the offending item: a line, or a place in a listing.</param>
 /// <param name="Message">What went wrong, phrased for a user rather than a developer.</param>
-public sealed record ParseError(int LineNumber, string Message)
+public sealed record ParseError(int Index, string Message)
 {
     /// <summary>
     /// Stable identifier for the kind of failure, or <see langword="null"/> when the message is
@@ -11,7 +11,7 @@ public sealed record ParseError(int LineNumber, string Message)
     /// </summary>
     /// <remarks>
     /// The domain stays language-free: it produces an English message plus a code, and the UI
-    /// translates <c>Parse.Error.{Code}</c> with <see cref="Arguments"/>, falling back to
+    /// translates <c>Collect.Error.{Code}</c> with <see cref="Arguments"/>, falling back to
     /// <see cref="Message"/>. Without this, a Russian UI would still print English error text —
     /// and moving the strings themselves into the domain would drag localisation down there.
     /// </remarks>
@@ -21,9 +21,9 @@ public sealed record ParseError(int LineNumber, string Message)
     public IReadOnlyList<object?> Arguments { get; init; } = [];
 
     /// <summary>Creates an error carrying a translation code.</summary>
-    public static ParseError Create(int lineNumber, string code, string message, params object?[] arguments) =>
-        new(lineNumber, message) { Code = code, Arguments = arguments };
+    public static ParseError Create(int index, string code, string message, params object?[] arguments) =>
+        new(index, message) { Code = code, Arguments = arguments };
 
     /// <inheritdoc />
-    public override string ToString() => $"Line {LineNumber}: {Message}";
+    public override string ToString() => $"#{Index}: {Message}";
 }