fix(naming): respect subfolder order when assigning audiobook part numbers (#1123)

## Summary

Audiobooks whose files are split across nested subfolders now keep their
on-disk folder order when "Rename and Organize" assigns part numbers.
Previously `natural_sort_key` keyed on `Path(path).name`, so identically
named files from different folders (`001.mp3` in `00_Introduction/` vs
`06_Side 6/`) collapsed together and the part sequence scrambled.

## Background

zazizou reported in #1007 that the total file count was right but
`06_Side 6/001.mp3` came out as part 1 instead of
`00_Introduction/001_About.mp3`. `scan.py` flattens every nested file
into one list and `assign_part_numbers` sorts it with
`natural_sort_key`, which discarded the parent-folder prefix. The fix
pads the existing `PAD_NUMBERS_PATTERN` numbers over the full relative
path (`str(path).lower()`) rather than the basename, so nested
audiobooks sort by folder first and then by filename. Flat directories
are unaffected because their files share a prefix, and
`natural_sort_key` is only called from `assign_part_numbers`, so nothing
else changes. I updated the test that pinned the old basename-only
ordering and added nested-subfolder and duplicate-basename cases;
`pytest tests/core/test_naming.py
tests/core/test_part_number_extraction.py` passes (102 tests).

Closes #1007

Co-authored-by: Matt Van Horn <455140+mvanhorn@users.noreply.github.com>

shelfmark/core/naming.py

@@ -91,8 +91,7 @@ PAD_NUMBERS_PATTERN = re.compile(r"\d+")
 
 def natural_sort_key(path: str | Path) -> str:
     """Generate a sort key with padded numbers for natural sorting."""
-    filename = Path(path).name.lower()
-    return PAD_NUMBERS_PATTERN.sub(lambda m: m.group().zfill(9), filename)
+    return PAD_NUMBERS_PATTERN.sub(lambda m: m.group().zfill(9), str(path).lower())
 
 
 def assign_part_numbers(

tests/core/test_part_number_extraction.py

@@ -43,10 +43,10 @@ class TestNaturalSortKey:
             "file10.mp3",
         ]
 
-    def test_uses_filename_only(self):
+    def test_uses_full_path(self):
         files = [Path("/z/dir/file1.mp3"), Path("/a/dir/file2.mp3")]
         sorted_files = sorted(files, key=natural_sort_key)
-        assert sorted_files[0].name == "file1.mp3"
+        assert sorted_files == [Path("/a/dir/file2.mp3"), Path("/z/dir/file1.mp3")]
 
 
 class TestAssignPartNumbers:
@@ -72,6 +72,23 @@ class TestAssignPartNumbers:
             (Path("Chapter 10.mp3"), "03"),
         ]
 
+    def test_nested_folders_are_numbered_in_folder_order(self):
+        files = [
+            Path("06_Side 6/002.mp3"),
+            Path("01_Side 1/001.mp3"),
+            Path("00_Introduction/001_About.mp3"),
+            Path("06_Side 6/001.mp3"),
+            Path("01_Side 1/002.mp3"),
+        ]
+
+        assert assign_part_numbers(files) == [
+            (Path("00_Introduction/001_About.mp3"), "01"),
+            (Path("01_Side 1/001.mp3"), "02"),
+            (Path("01_Side 1/002.mp3"), "03"),
+            (Path("06_Side 6/001.mp3"), "04"),
+            (Path("06_Side 6/002.mp3"), "05"),
+        ]
+
     def test_custom_zero_padding(self):
         files = [Path("a.mp3"), Path("b.mp3")]
         assert assign_part_numbers(files, zero_pad_width=3) == [
@@ -190,7 +207,10 @@ class TestEdgeCases:
     def test_identical_filenames_different_dirs(self):
         files = [Path("/dir2/track.mp3"), Path("/dir1/track.mp3")]
         result = assign_part_numbers(files)
-        assert len(result) == 2
+        assert result == [
+            (Path("/dir1/track.mp3"), "01"),
+            (Path("/dir2/track.mp3"), "02"),
+        ]
 
     def test_unicode_filenames(self):
         files = [Path("日本語タイトル 02.mp3"), Path("日本語タイトル 01.mp3")]