feat: scan.resolve_index with end-of-batch error

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

gates/scan.py

@@ -28,3 +28,11 @@ def list_images(folder, depth=0):
                 results.append(str(cur / name))
     results.sort(key=lambda p: natural_key(os.path.relpath(p, root)))
     return results
+
+
+def resolve_index(count, index):
+    if count == 0:
+        raise FileNotFoundError("No images found in folder")
+    if index < 0 or index >= count:
+        raise IndexError(f"index {index} out of range: {count} images")
+    return index

tests/test_scan.py

@@ -37,3 +37,19 @@ def test_list_images_bad_path_raises(tmp_path):
     import pytest
     with pytest.raises(NotADirectoryError):
         scan.list_images(str(tmp_path / "nope"))
+
+def test_resolve_index_ok():
+    assert scan.resolve_index(5, 0) == 0
+    assert scan.resolve_index(5, 4) == 4
+
+def test_resolve_index_out_of_range_raises():
+    import pytest
+    with pytest.raises(IndexError):
+        scan.resolve_index(5, 5)
+    with pytest.raises(IndexError):
+        scan.resolve_index(5, -1)
+
+def test_resolve_index_empty_raises():
+    import pytest
+    with pytest.raises(FileNotFoundError):
+        scan.resolve_index(0, 0)