fix: prioritize Structure A over Structure B in library structure detection

When a top-level `roms/` folder exists (Structure A), never detect the
library as Structure B, even if individual `<platform>/roms/` directories
also exist. This prevents existing Structure A libraries from being broken
after upgrading to 4.9.0.

- `has_structure_path_b` in `config_manager.py` now returns `False` early
  when `{LIBRARY_BASE_PATH}/{ROMS_FOLDER_NAME}` is an existing directory
- `detect_library_structure()` in `platforms_handler.py` now explicitly
  checks Structure A (`os.path.exists(roms_path)`) before consulting
  `cnfg.has_structure_path_b`
- Updated test to assert Structure A wins when both layouts coexist

Co-authored-by: gantoine <3247106+gantoine@users.noreply.github.com>

backend/config/config_manager.py

@@ -140,6 +140,13 @@ class Config:
 
     @functools.cached_property
     def has_structure_path_b(self) -> bool:
+        # Structure A (roms/{platform}) takes priority: if the top-level roms
+        # folder exists, never claim Structure B even if some platform dirs
+        # happen to contain a roms sub-folder.
+        roms_path = os.path.join(LIBRARY_BASE_PATH, self.ROMS_FOLDER_NAME)
+        if os.path.isdir(roms_path):
+            return False
+
         pattern = os.path.join(
             LIBRARY_BASE_PATH, "*", glob.escape(self.ROMS_FOLDER_NAME)
         )

backend/handler/filesystem/platforms_handler.py

@@ -33,22 +33,27 @@ class FSPlatformsHandler(FSHandler):
     def detect_library_structure(self) -> LibraryStructure | None:
         """Detects the library structure type.
 
+        Structure A (roms/{platform}) takes priority over Structure B
+        ({platform}/roms) so that existing libraries are not broken when a
+        stray {platform}/roms directory happens to exist alongside them.
+
         Returns:
-            "LibraryStructure.B" for Structure B ({platform}/roms) when any
-                platform has a roms subfolder.
             "LibraryStructure.A" for Structure A (roms/{platform}) when the
                 top-level roms folder exists.
+            "LibraryStructure.B" for Structure B ({platform}/roms) when no
+                top-level roms folder exists but at least one platform has a
+                roms subfolder.
             None if no structure detected.
         """
         cnfg = cm.get_config()
 
-        if cnfg.has_structure_path_b:
-            return LibraryStructure.B
-
         roms_path = os.path.join(LIBRARY_BASE_PATH, cnfg.ROMS_FOLDER_NAME)
         if os.path.exists(roms_path):
             return LibraryStructure.A
 
+        if cnfg.has_structure_path_b:
+            return LibraryStructure.B
+
         return None
 
     def get_platforms_directory(self) -> str:

backend/tests/handler/filesystem/test_platforms_handler.py

@@ -337,13 +337,15 @@ class TestFSPlatformsHandler:
         with patch(
             "handler.filesystem.platforms_handler.cm.get_config", return_value=config
         ):
-            result = handler.detect_library_structure()
-            assert result == LibraryStructure.B
+            with patch("os.path.exists", return_value=False):
+                result = handler.detect_library_structure()
+                assert result == LibraryStructure.B
 
-    def test_detect_library_structure_b_takes_priority_over_a(
+    def test_detect_library_structure_a_takes_priority_over_b(
         self, handler: FSPlatformsHandler, config
     ):
-        """Structure B is reported even when the top-level roms folder exists."""
+        """Structure A is reported when the top-level roms folder exists, even
+        when Structure B directories are also present."""
         config.has_structure_path_b = True
 
         with patch(
@@ -351,7 +353,7 @@ class TestFSPlatformsHandler:
         ):
             with patch("os.path.exists", return_value=True):
                 result = handler.detect_library_structure()
-                assert result == LibraryStructure.B
+                assert result == LibraryStructure.A
 
     def test_detect_library_structure_none(self, handler: FSPlatformsHandler, config):
         """Test detect_library_structure returns None when no structure detected"""