feat: add build_mask_output_dir utility

tests/test_utils.py

@@ -1,5 +1,5 @@
 import tempfile, os
-from main import build_export_path, format_time, build_ffmpeg_command
+from main import build_export_path, format_time, build_ffmpeg_command, build_mask_output_dir
 from main import _normalize_filename, ProcessedDB
 
 
@@ -173,3 +173,12 @@ def test_ffmpeg_command_portrait_and_resize():
 def test_ffmpeg_command_portrait_off():
     cmd = build_ffmpeg_command("/in/video.mp4", 0.0, "/out/clip.mp4")
     assert "-vf" not in cmd
+
+def test_mask_output_dir_basic():
+    assert build_mask_output_dir("/out/clip_001.mp4") == "/out/clip_001_masks"
+
+def test_mask_output_dir_mkv():
+    assert build_mask_output_dir("/out/my_clip.mkv") == "/out/my_clip_masks"
+
+def test_mask_output_dir_nested():
+    assert build_mask_output_dir("/a/b/c/shot_042.mp4") == "/a/b/c/shot_042_masks"