feat(identify): detect China TC260 AIGC label (Doubao et al.)

China-served generators embed an XMP <TC260:AIGC>{"Label":"1",...} block
(China's mandatory AI-content labeling, TC260 standard). Doubao (ByteDance)
uses it -- verified on the real #13 sample. It's none of C2PA / SynthID /
imwatermark / IPTC, so identify() previously returned unknown.

- metadata: AIGC_MARKERS + aigc_label() (json-decodes the HTML-entity-encoded
  block); has_ai_metadata + get_ai_metadata now surface it.
- identify: new 'aigc' signal -> is_ai True, platform 'China AIGC-labeled
  generator (TC260; e.g. Doubao)', carries the ContentProducer code.
- Container-agnostic raw-byte scan, so it covers the whole China-AIGC ecosystem
  (Jimeng/Kling/Qwen/Ernie share the standard).
- Tests: synthetic TC260 block (metadata + identify). Docs updated.

Addresses #13.

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

tests/test_identify.py

@@ -317,3 +317,35 @@ class TestIdentifyInvisibleWatermark:
     def test_check_invisible_false_skips(self, tmp_path: Path):
         r = identify(self._sdxl_watermarked(tmp_path), check_visible=False, check_invisible=False)
         assert not any(s.name == "invisible_watermark" for s in r.signals)
+
+
+class TestIdentifyAIGC:
+    """China TC260 AIGC label is detected and attributed (e.g. Doubao)."""
+
+    def _aigc_png(self, tmp_path: Path) -> Path:
+        from PIL import Image
+
+        p = tmp_path / "doubao.png"
+        Image.new("RGB", (32, 32)).save(p)
+        xmp = (
+            '<x:xmpmeta xmlns:x="adobe:ns:meta/"><rdf:RDF '
+            'xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#">'
+            '<rdf:Description xmlns:TC260="http://www.tc260.org.cn/ns/AIGC/1.0/">'
+            "<TC260:AIGC>{&quot;Label&quot;:&quot;1&quot;,&quot;ContentProducer&quot;:&quot;BYTEDANCE001&quot;}"
+            "</TC260:AIGC></rdf:Description></rdf:RDF></x:xmpmeta>"
+        )
+        with open(p, "ab") as f:
+            f.write(xmp.encode())
+        return p
+
+    def test_aigc_detected(self, tmp_path: Path):
+        r = identify(self._aigc_png(tmp_path), check_visible=False)
+        assert r.is_ai_generated is True
+        assert r.platform is not None
+        assert "AIGC" in r.platform or "TC260" in r.platform
+        assert any("AIGC" in w for w in r.watermarks)
+
+    def test_aigc_signal_carries_producer(self, tmp_path: Path):
+        r = identify(self._aigc_png(tmp_path), check_visible=False)
+        sig = next(s for s in r.signals if s.name == "aigc")
+        assert "BYTEDANCE001" in sig.detail

tests/test_metadata.py

@@ -390,3 +390,46 @@ class TestExifGenerator:
 
     def test_clean_image_is_none(self, tmp_clean_png: Path):
         assert exif_generator(tmp_clean_png) is None
+
+
+class TestAIGCLabel:
+    """China TC260 AIGC labeling (Doubao and other China-served generators)."""
+
+    def _aigc_png(self, tmp_path: Path, label: str = "1", producer: str = "TESTPRODUCER001") -> Path:
+        from remove_ai_watermarks.metadata import aigc_label  # noqa: F401  (import-time guard)
+
+        p = tmp_path / "doubao.png"
+        Image.new("RGB", (32, 32)).save(p)
+        # XMP is HTML-entity encoded in real files; aigc_label must unescape it.
+        xmp = (
+            '<x:xmpmeta xmlns:x="adobe:ns:meta/"><rdf:RDF '
+            'xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#">'
+            '<rdf:Description rdf:about="" '
+            'xmlns:TC260="http://www.tc260.org.cn/ns/AIGC/1.0/"><TC260:AIGC>'
+            f"{{&quot;Label&quot;:&quot;{label}&quot;,&quot;ContentProducer&quot;:&quot;{producer}&quot;}}"
+            "</TC260:AIGC></rdf:Description></rdf:RDF></x:xmpmeta>"
+        )
+        with open(p, "ab") as f:
+            f.write(xmp.encode())
+        return p
+
+    def test_parses_label_and_producer(self, tmp_path: Path):
+        from remove_ai_watermarks.metadata import aigc_label
+
+        info = aigc_label(self._aigc_png(tmp_path))
+        assert info is not None
+        assert info["Label"] == "1"
+        assert info["ContentProducer"] == "TESTPRODUCER001"
+
+    def test_none_when_absent(self, tmp_clean_png):
+        from remove_ai_watermarks.metadata import aigc_label
+
+        assert aigc_label(tmp_clean_png) is None
+
+    def test_has_ai_metadata_detects_aigc(self, tmp_path: Path):
+        assert has_ai_metadata(self._aigc_png(tmp_path))
+
+    def test_get_ai_metadata_surfaces_aigc(self, tmp_path: Path):
+        meta = get_ai_metadata(self._aigc_png(tmp_path))
+        assert "aigc_label" in meta
+        assert "TC260" in meta["aigc_label"]