fix(test): threshold test needs structured images (solid colors phash-collapse)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/test_phash_dedup.py

@@ -52,6 +52,21 @@ def _write(path: Path, color, size):
     Image.new("RGB", size, color).save(path)
 
 
+def _write_split(path: Path, orient, size):
+    """Structured (half black / half white) image — solid colors all
+    phash-collapse to the same value, so a real threshold test needs DCT
+    content. Vertical vs horizontal split = structurally far apart."""
+    path.parent.mkdir(parents=True, exist_ok=True)
+    w, h = size
+    im = Image.new("L", size, 0)
+    px = im.load()
+    for y in range(h):
+        for x in range(w):
+            if (x / w if orient == "v" else y / h) >= 0.5:
+                px[x, y] = 255
+    im.convert("RGB").save(path)
+
+
 def test_non_similar_imports_with_phash(importer, import_layout):
     import_root, _ = import_layout
     src = import_root / "a.png"
@@ -127,15 +142,18 @@ def test_smaller_existing_is_superseded(importer, import_layout):
 
 
 def test_threshold_controls_match(importer, import_layout):
+    # Structurally distinct images (orthogonal splits) are far apart in
+    # phash space, so a tight threshold keeps them independent rather than
+    # collapsing the larger one into a supersede.
     import_root, _ = import_layout
     _set_threshold(importer, 0)
     a = import_root / "a.png"
-    _write(a, (100, 100, 100), (200, 200))
+    _write_split(a, "v", (200, 200))
     importer.import_one(a)
     b = import_root / "b.png"
-    _write(b, (104, 100, 100), (900, 900))  # tiny tweak
+    _write_split(b, "h", (900, 900))  # different structure, larger
     r = importer.import_one(b)
-    assert r.status == "imported"  # threshold 0 → not treated as near-dup
+    assert r.status == "imported"  # threshold 0 + far → independent import
 
 
 def test_import_task_maps_superseded_to_complete_and_requeues():