Look for the simulator when the picked one is not in this installation

The pick is kept in the per-user eu07.ini, which every installation shares,
so a binary chosen in one folder was missing in another and the starter
complained although a simulator sat right there. A missing pick now gives
way to the search and is logged once.

StarterNG.Tests/ExecutableLocatorTests.cs

@@ -68,6 +68,31 @@ public class ExecutableLocatorTests
         Assert.Equal(ExeProblem.None, problem);
     }
 
+    [Fact]
+    public void A_pick_from_another_installation_gives_way_to_the_search()
+    {
+        var files = new InMemoryFileSystem().WithExecutable(TestInstallation.At("eu07"), Elf);
+        var installation = new TestInstallation(files);
+        var settings = new SimulatorSettings { SelectExeAutomatically = false, ExecutablePath = "eu07-elsewhere" };
+
+        string resolved = installation.Executables.Resolve(settings, out var problem);
+
+        Assert.EndsWith("eu07", resolved);
+        Assert.Equal(ExeProblem.None, problem);
+    }
+
+    [Fact]
+    public void A_missing_pick_is_reported_when_the_installation_has_none_either()
+    {
+        var installation = new TestInstallation();
+        var settings = new SimulatorSettings { SelectExeAutomatically = false, ExecutablePath = "eu07-elsewhere" };
+
+        string resolved = installation.Executables.Resolve(settings, out var problem);
+
+        Assert.Equal("eu07-elsewhere", resolved);
+        Assert.Equal(ExeProblem.NotFound, problem);
+    }
+
     [Fact]
     public void Data_files_beside_the_binary_are_never_offered_as_candidates()
     {

StarterNG/Infrastructure/Settings/ExecutableLocator.cs

@@ -21,6 +21,7 @@ public sealed class ExecutableLocator
 
     private string? _wine;
     private bool _wineLookedUp;
+    private string? _missingPickLogged;
 
     public ExecutableLocator(IFileSystem files, IGamePaths paths, IEnvironment environment, IDiagnosticsLog log)
     {
@@ -32,14 +33,39 @@ public sealed class ExecutableLocator
 
     public string CanonicalName => _environment.IsWindows ? "eu07.exe" : "eu07";
 
+    /// <summary>
+    /// The simulator to start: the one picked in the settings, or the best one in
+    /// the installation. A pick that is not there - the settings live per user and
+    /// are shared by every installation, so it may come from another one - gives
+    /// way to the search rather than stopping the start.
+    /// </summary>
     public string Resolve(SimulatorSettings settings, out ExeProblem problem)
     {
         if (!settings.SelectExeAutomatically && !string.IsNullOrWhiteSpace(settings.ExecutablePath))
         {
             problem = Validate(FullPath(settings.ExecutablePath));
-            return settings.ExecutablePath;
+            if (problem != ExeProblem.NotFound)
+                return settings.ExecutablePath;
+
+            string found = Search(out var searched);
+            if (searched == ExeProblem.NotFound)
+                return settings.ExecutablePath;
+
+            // Resolved on every start and for the title bar, so said once.
+            if (_missingPickLogged != settings.ExecutablePath)
+            {
+                _log.Log($"The chosen simulator {settings.ExecutablePath} is not in {_paths.Root}, using {found}");
+                _missingPickLogged = settings.ExecutablePath;
+            }
+            problem = searched;
+            return found;
         }
 
+        return Search(out problem);
+    }
+
+    private string Search(out ExeProblem problem)
+    {
         string? fallback = null;
         try
         {