Introduce layers, ports and a composition root

Ports in Application/Abstractions, adapters in Infrastructure, hand-wired
AppServices (no reflection, as PublishAot requires). Adds a test project.

StarterNG/Application/Abstractions/IClock.cs

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+namespace StarterNG.Application.Abstractions;
+
+using System;
+
+/// <summary>
+/// Port over the system clock. Scenery weather defaults, timetable rendering and
+/// log timestamps all depend on "now"; routing them through a clock keeps those
+/// rules deterministic under test.
+/// </summary>
+public interface IClock
+{
+    DateTime Now { get; }
+
+    DateTime UtcNow { get; }
+}

StarterNG/Application/Abstractions/IDiagnosticsLog.cs

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+using System;
+using System.Collections.Generic;
+
+namespace StarterNG.Application.Abstractions;
+
+/// <summary>
+/// Port over the starter's error log. Domain and application code depends on
+/// this interface only; the file-backed implementation and the message box that
+/// surfaces faults to the user live in Infrastructure and Presentation.
+/// </summary>
+public interface IDiagnosticsLog
+{
+    void Log(string message);
+
+    void Log(string context, Exception exception);
+
+    void Log(IEnumerable<string> messages);
+
+    /// <summary>Everything logged so far, newline separated.</summary>
+    string Text { get; }
+
+    void Clear();
+
+    void Flush();
+}

StarterNG/Application/Abstractions/IFileSystem.cs

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+using System;
+using System.Collections.Generic;
+using System.IO;
+using System.Text;
+
+namespace StarterNG.Application.Abstractions;
+
+/// <summary>
+/// Port over the file system. Everything that reads or writes the MaSzyna
+/// installation goes through this interface instead of <see cref="File"/> and
+/// <see cref="Directory"/>, so parsers and use cases can be exercised against an
+/// in-memory installation in tests.
+/// </summary>
+/// <remarks>
+/// Kept deliberately narrow: only the operations the application actually
+/// performs are exposed. Paths are passed through verbatim to the adapter, which
+/// resolves them relative to the process working directory just like the
+/// framework methods it replaces.
+/// </remarks>
+public interface IFileSystem
+{
+    bool FileExists(string path);
+
+    bool DirectoryExists(string path);
+
+    string ReadAllText(string path);
+
+    string ReadAllText(string path, Encoding encoding);
+
+    string[] ReadAllLines(string path);
+
+    Stream OpenRead(string path);
+
+    void WriteAllText(string path, string contents);
+
+    void WriteAllText(string path, string contents, Encoding encoding);
+
+    void WriteAllLines(string path, IEnumerable<string> lines);
+
+    void WriteAllBytes(string path, byte[] bytes);
+
+    void CreateDirectory(string path);
+
+    void DeleteFile(string path);
+
+    void CopyFile(string sourcePath, string destinationPath, bool overwrite);
+
+    /// <summary>Top-level files, empty when the directory does not exist.</summary>
+    IReadOnlyList<string> GetFiles(string path, string searchPattern = "*");
+
+    /// <summary>Top-level directories, empty when the directory does not exist.</summary>
+    IReadOnlyList<string> GetDirectories(string path);
+
+    DateTime GetLastWriteTimeUtc(string path);
+}

StarterNG/Application/Abstractions/IGamePaths.cs

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+namespace StarterNG.Application.Abstractions;
+
+/// <summary>
+/// The layout of a MaSzyna installation. Replaces the
+/// <c>Directory.GetCurrentDirectory()</c> calls and hard-coded "scenery/",
+/// "dynamic", "textures/mini/" literals that were previously spread across
+/// parsers, views and the loader.
+/// </summary>
+public interface IGamePaths
+{
+    /// <summary>Installation root: the folder holding eu07.exe.</summary>
+    string Root { get; }
+
+    /// <summary>Scenery definitions (*.scn).</summary>
+    string Scenery { get; }
+
+    /// <summary>Rolling stock: dynamic/&lt;maker&gt;/&lt;vehicle&gt;/textures.txt.</summary>
+    string Dynamic { get; }
+
+    /// <summary>Vehicle thumbnails used by the depot and the consist strip.</summary>
+    string MiniTextures { get; }
+
+    /// <summary>Simulator data files (load_weights.txt and friends).</summary>
+    string Data { get; }
+
+    /// <summary>Starter-owned configuration: presets, profiles, key bindings.</summary>
+    string StarterConfig { get; }
+
+    /// <summary>The starter's own error log (starter/bledy.txt).</summary>
+    string DiagnosticsLog { get; }
+
+    /// <summary>Resolves a path relative to <see cref="Root"/>.</summary>
+    string FromRoot(params string[] segments);
+}

StarterNG/Application/Abstractions/ILocalizedStrings.cs

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+namespace StarterNG.Application.Abstractions;
+
+/// <summary>
+/// Port over the translation table. Anything that needs a user-facing string
+/// takes this, rather than reaching for the concrete localization service or a
+/// static on <c>App</c>.
+/// </summary>
+public interface ILocalizedStrings
+{
+    /// <summary>
+    /// Translation for a key. Missing keys return the key itself (and are logged
+    /// once), so an untranslated screen stays usable.
+    /// </summary>
+    string this[string key] { get; }
+
+    /// <summary>Lookup for a key that may legitimately be absent; "" when missing.</summary>
+    string Opt(string key);
+
+    string CurrentLanguage { get; }
+
+    string CurrentLangCode { get; }
+}

StarterNG/Application/Abstractions/IProcessLauncher.cs

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+using System.Collections.Generic;
+using System.Threading;
+using System.Threading.Tasks;
+
+namespace StarterNG.Application.Abstractions;
+
+/// <summary>
+/// A process the starter has launched or adopted. Exposes only what the starter
+/// needs — whether the simulator is still up, and a way to await its exit — so
+/// the presentation layer never handles <see cref="System.Diagnostics.Process"/>
+/// directly.
+/// </summary>
+public interface IProcessHandle
+{
+    bool HasExited { get; }
+
+    Task WaitForExitAsync(CancellationToken cancellationToken = default);
+}
+
+/// <summary>
+/// Port over process creation: starting the simulator, opening a file, folder or
+/// URL in the desktop shell, and adopting an already running simulator.
+/// </summary>
+public interface IProcessLauncher
+{
+    /// <summary>
+    /// Starts an executable with the given arguments. Returns null when the
+    /// process could not be created; <paramref name="error"/> then carries the
+    /// reason for display.
+    /// </summary>
+    IProcessHandle? Start(string executablePath, IReadOnlyList<string> arguments, string? workingDirectory,
+                          out string? error);
+
+    /// <summary>
+    /// Hands a path or URL to the desktop shell (explorer, xdg-open, ...).
+    /// Returns false and logs when the shell refused it.
+    /// </summary>
+    bool OpenInShell(string pathOrUrl);
+
+    /// <summary>The first running process with that name, or null.</summary>
+    IProcessHandle? FindRunning(string processName);
+}