feat(c2pa): recognize Stability AI issuer; fix Microsoft platform label

Collected live C2PA positives from Bing Image Creator and Stability Brand
Studio (DreamStudio successor) and learned two things our scan got wrong:

- Bing now runs Microsoft's own MAI-Image model, not DALL-E, and signs
  C2PA as 'Microsoft'. The scan caught it, but the platform label claimed
  'Microsoft Designer (DALL-E / OpenAI backend)'. Relabeled model-neutral:
  'Microsoft (Bing Image Creator / Designer)'.
- Stability signs C2PA as 'Stability AI' (cert 'Stability AI Ltd'), which
  was not in C2PA_ISSUERS, so it read as 'unknown signer'. Added the issuer
  and a platform mapping. Stability uses no SynthID and (on its current
  Stable Image model) no imwatermark watermark -- verified, both negative.

Both ingested as SynthID-negative corpus fixtures (they are AI but not
SynthID) for issuer-coverage. Canva skipped: its downloads are re-encoded
design exports that strip C2PA, so a Canva sample would be inconclusive.

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

tests/test_identify.py

@@ -51,6 +51,17 @@ class TestAttributePlatform:
         assert platform
         assert "signer" in platform.lower()
 
+    def test_microsoft_label_is_model_neutral(self):
+        # Bing now runs MAI-Image, not DALL-E; the label must not claim DALL-E.
+        platform = _attribute_platform(["Microsoft"])
+        assert platform
+        assert "DALL-E" not in platform
+
+    def test_stability(self):
+        platform = _attribute_platform(["Stability AI"])
+        assert platform
+        assert "Stability AI" in platform
+
     def test_empty_is_none(self):
         assert _attribute_platform([]) is None
 
@@ -100,6 +111,14 @@ class TestIdentifyNonPng:
         r = identify(path, check_visible=False)
         assert any("SynthID" in w for w in r.watermarks)
 
+    def test_stability_ai_issuer_attributed_no_synthid(self, tmp_path: Path):
+        path = self._c2pa_jpeg(tmp_path, b"Stability AI ... trainedAlgorithmicMedia")
+        r = identify(path, check_visible=False)
+        assert r.is_ai_generated is True
+        assert r.platform is not None
+        assert "Stability AI" in r.platform
+        assert not any("SynthID" in w for w in r.watermarks)  # Stability does not use SynthID
+
     def test_c2pa_without_ai_marker_is_unknown(self, tmp_path: Path):
         # Adobe signs C2PA on plain Photoshop edits too. Without an AI digital-
         # source marker, the honest verdict is unknown -- the C2PA watermark is