feat(engines): discover MSYS2 tools as a fallback

docs/ENGINES.md

@@ -75,7 +75,8 @@ the same on every PC. An external engine is looked for in this order:
 1. a copy the user picked in Settings → *Engines*;
 2. Filee's own copy (bundled or downloaded);
 3. a copy already on the computer: on `PATH`, in Homebrew's and the usual Unix folders, or where the LibreOffice,
-   calibre, Ghostscript and Tesseract installers put it. Settings → *Engines* shows it as "Found on this PC", with a note when
+   calibre, Ghostscript and Tesseract installers put it. On Windows, FFmpeg, Tesseract and Whisper are also looked for in
+   the native environments under the system drive's `msys64` folder, after these locations. Settings → *Engines* shows it as "Found on this PC", with a note when
    its version is older than the one Filee downloads.
 
 Where Filee has no download of an engine (FFmpeg, Ghostscript and Tesseract on macOS, LibreOffice, Ghostscript and

src/Filee.Engines/Infrastructure/EngineEnvironment.cs

@@ -256,12 +256,21 @@ private static IEnumerable<string> SystemFolders(string id)
                     break;
             }
         }
+        // MSYS2 is an optional fallback after PATH and the usual installer locations, never ahead of Filee's copies.
+        if (OperatingSystem.IsWindows() && Path.GetPathRoot(Environment.SystemDirectory) is { Length: > 0 } drive)
+            folders.AddRange(Msys2Folders(id, Path.Combine(drive, "msys64")));
         // Never Filee's own folders: those are its own copy, found before.
         var comparison = OperatingSystem.IsWindows() ? StringComparison.OrdinalIgnoreCase : StringComparison.Ordinal;
         return folders.Distinct(OperatingSystem.IsWindows() ? StringComparer.OrdinalIgnoreCase : StringComparer.Ordinal)
             .Where(f => !IsWithin(f, DownloadRoot, comparison) && !IsWithin(f, AppContext.BaseDirectory, comparison));
     }
 
+    /// <summary>Native MSYS2 environments under a standard installation; do not search MSYS's own Unix tools.</summary>
+    internal static IEnumerable<string> Msys2Folders(string id, string root) =>
+        id is "ffmpeg" or "tesseract" or "whisper"
+            ? new[] { "ucrt64", "clang64", "mingw64", "clangarm64", "mingw32" }.Select(env => Path.Combine(root, env, "bin"))
+            : [];
+
     /// <summary>Where LibreOffice's Windows installer records its program folder (the key's default value).</summary>
     internal const string LibreOfficeInstallPathKey = @"SOFTWARE\LibreOffice\UNO\InstallPath";
 

tests/Filee.Engines.Tests/PortableEngineTests.cs

@@ -91,6 +91,27 @@ public void Unix_FFmpeg_requires_both_tools_in_the_same_folder()
         Assert.Equal(_root, EngineEnvironment.FolderWithPrograms("ffmpeg", _root, windows: false));
     }
 
+    [Theory]
+    [InlineData("ffmpeg", "mingw64", "ffmpeg.exe", "ffprobe.exe")]
+    [InlineData("tesseract", "clang64", "tesseract.exe", null)]
+    [InlineData("whisper", "ucrt64", "whisper-cli.exe", null)]
+    public void Msys2_native_engines_are_found_only_when_the_required_programs_exist(
+        string id, string environment, string program, string? second)
+    {
+        var bin = Path.Combine(_root, environment, "bin");
+        Directory.CreateDirectory(bin);
+        File.WriteAllText(Path.Combine(bin, program), "binary");
+        var folders = EngineEnvironment.Msys2Folders(id, _root).ToList();
+        if (second is not null)
+        {
+            Assert.Null(folders.Select(path => EngineEnvironment.FolderWithPrograms(id, path, windows: true)).FirstOrDefault(path => path is not null));
+            File.WriteAllText(Path.Combine(bin, second), "binary");
+        }
+        Assert.Equal(bin, folders.Select(path => EngineEnvironment.FolderWithPrograms(id, path, windows: true)).FirstOrDefault(path => path is not null));
+        Assert.DoesNotContain(Path.Combine(_root, "usr", "bin"), folders);
+        Assert.Empty(EngineEnvironment.Msys2Folders("pandoc", _root));
+    }
+
     [Fact]
     public async Task Tar_archives_preserve_program_layout_contents_and_Unix_executable_mode()
     {