Scaffold AvParser: Avalonia 12 shell with adaptive layout

Greenfield skeleton for a parser desktop app. The domain is deliberately a
placeholder — IParser<TIn,TOut> plus two sample parsers — so the shell is
runnable and verifiable end to end before real logic lands.

Layers run one way: Core (no Avalonia, no IO) <- Infrastructure <- UI <- Desktop.
UI is a class library rather than the exe so headless tests build real views
without dragging in Program.cs, Serilog or the container.

Adaptive layout is built from what Avalonia actually offers, since it has no
AdaptiveTrigger or media queries: ResponsiveLayout observes Visual.Bounds and
projects a breakpoint onto both an attached property and :compact/:medium/
:expanded pseudoclasses, with 24px hysteresis so dragging a window edge cannot
make the layout flap. Pane state lives in the view model because a style setter
loses to a local value permanently; styles own only the visual variance.

Stack notes worth remembering: Avalonia.ReactiveUI is deprecated in favour of
ReactiveUI.Avalonia, and ReactiveUI 24 runs on the Primitives engine (RxVoid,
ISequencer, Signal<T>) and no longer self-initialises. Avalonia.Headless.XUnit
12.x requires xUnit v3. InvariantGlobalization must stay false or Semi.Avalonia
throws in its static constructor.

102 tests across three projects, including headless guards for the two failures
that are otherwise completely silent: a stylesheet whose selectors match nothing,
and a light palette too low-contrast for cards to read.

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

src/AvParser.Core/Parsing/ParseOutcome.cs

@@ -0,0 +1,40 @@
+using System.Diagnostics.CodeAnalysis;
+
+namespace AvParser.Core.Parsing;
+
+/// <summary>
+/// Result of parsing a single record: either a value or a recoverable <see cref="ParseError"/>.
+/// </summary>
+/// <remarks>
+/// A struct rather than a class hierarchy: parsers emit one of these per line, and on a
+/// million-line input the allocation difference is the whole cost of the parse.
+/// </remarks>
+public readonly record struct ParseOutcome<T>
+{
+    private ParseOutcome(T? value, ParseError? error)
+    {
+        Value = value;
+        Error = error;
+    }
+
+    /// <summary>The parsed value, or <see langword="null"/> when <see cref="IsSuccess"/> is false.</summary>
+    public T? Value { get; }
+
+    /// <summary>The failure, or <see langword="null"/> when <see cref="IsSuccess"/> is true.</summary>
+    public ParseError? Error { get; }
+
+    /// <summary><see langword="true"/> when a value was produced.</summary>
+    [MemberNotNullWhen(false, nameof(Error))]
+    public bool IsSuccess => Error is null;
+
+    /// <summary>Creates a successful outcome.</summary>
+    public static ParseOutcome<T> Success(T value) => new(value, null);
+
+    /// <summary>Creates a failed outcome.</summary>
+    public static ParseOutcome<T> Failure(ParseError error) =>
+        new(default, error ?? throw new ArgumentNullException(nameof(error)));
+
+    /// <summary>Creates a failed outcome from its parts.</summary>
+    public static ParseOutcome<T> Failure(int lineNumber, string message) =>
+        Failure(new ParseError(lineNumber, message));
+}