perf(identify): decode the image once for all visible-mark detectors

identify(check_visible=True) ran the Gemini-sparkle detector and the
Doubao/Jimeng text-mark detector each with its own image_io.imread, so the
same bitmap was fully decoded twice. On a memory-constrained host (the raiw.cc
512 MB web worker, which runs identify on every upload) that doubled the peak
decode allocation and contributed to OOM restarts.

Decode once in identify() and pass the BGR array to both detectors. The detect
methods already accept an NDArray, so this only threads the pre-decoded array
through: detect_sparkle_confidence and the two _visible_* helpers gain an
optional image= param that, when None, preserves the old self-read behavior
(so direct callers and the cv2-missing/unreadable paths are unchanged).

Only the visible path is deduplicated; the optional check_invisible decoders
are unaffected (and off on the web hot path). Adds a test asserting
identify(check_visible=True, check_invisible=False) decodes exactly once.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/test_identify.py

@@ -434,6 +434,18 @@ class TestIdentifyVisibleTextMarks:
             r = identify(tmp_png_with_ai_metadata, check_visible=True)
         assert r.confidence == "high"
 
+    def test_visible_path_decodes_file_once(self, tmp_clean_png: Path):
+        """The web path identify(check_visible=True, check_invisible=False) must
+        decode the image exactly once and share the array across the sparkle +
+        text-mark detectors. Two decodes of the same bitmap spiked memory on the
+        small web worker (the OOM the decode-once refactor addresses)."""
+        import remove_ai_watermarks.image_io as image_io
+
+        real_imread = image_io.imread
+        with patch.object(image_io, "imread", side_effect=real_imread) as mock_imread:
+            identify(tmp_clean_png, check_visible=True, check_invisible=False)
+        assert mock_imread.call_count == 1
+
 
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