feat(identify): attribute Canva and BytePlus C2PA; fix BytePlus->Adobe mislabel

Mining the local production corpus (25,725 imgs) surfaced two AI vendors signing
C2PA that the registry missed:
- Canva (Magic Media) signed "Canva" + trainedAlgorithmicMedia -> detected AI but
  no platform attributed (disproves the old "Canva exports strip C2PA" assumption).
- BytePlus (ByteDance international: Seedream/Seededit) signs "Byteplus Pte. Ltd.";
  the bare volcengine needle missed it, so its output was mis-attributed to "Adobe
  Firefly" via an incidental "Adobe XMP" string the fallback byte-scan picked up.

Adding both to C2PA_AI_VENDORS lets the clean manifest issuer attribute them
directly. Corpus re-run: 16 platform changes, all improvements (3 Adobe->ByteDance
fixes, 4 None/TC260->ByteDance, 9 None->Canva), 0 regressions. An attempted
signer-based attribution fallback was measured and dropped: it regressed 18 images
(friendly ByteDance label -> raw Chinese cert org; IPTC tool name pre-empted).

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

tests/test_identify.py

@@ -66,6 +66,19 @@ class TestAttributePlatform:
         assert platform
         assert "Stability AI" in platform
 
+    def test_canva(self):
+        platform = _attribute_platform(["Canva"])
+        assert platform
+        assert "Canva" in platform
+
+    def test_byteplus_attributes_to_bytedance(self):
+        # ByteDance's intl brand signs as "Byteplus Pte. Ltd."; the registry maps
+        # it to the ByteDance platform (was mis-read as Adobe via an incidental
+        # "Adobe XMP" file string before the entry existed).
+        platform = _attribute_platform(["BytePlus (ByteDance)"])
+        assert platform
+        assert "ByteDance" in platform
+
     def test_empty_is_none(self):
         assert _attribute_platform([]) is None