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>

src/remove_ai_watermarks/identify.py

@@ -50,10 +50,13 @@ _SPARKLE_THRESHOLD = 0.5
 # manifest names several issuers (Microsoft Designer signs as "OpenAI,
 # Microsoft"), the first match wins so the product, not the backend, is named.
 _ISSUER_PLATFORM: tuple[tuple[str, str], ...] = (
-    ("Microsoft", "Microsoft Designer (DALL-E / OpenAI backend)"),
+    # Microsoft signs both Designer and Bing Image Creator; Bing now runs its
+    # own MAI-Image model (not DALL-E), so the label stays model-neutral.
+    ("Microsoft", "Microsoft (Bing Image Creator / Designer)"),
     ("Adobe", "Adobe Firefly"),
     ("OpenAI", "OpenAI (ChatGPT / gpt-image / DALL-E / Sora)"),
     ("Google", "Google (Gemini / Imagen)"),
+    ("Stability AI", "Stability AI (Stable Image / DreamStudio)"),
 )
 
 # PNG-text / EXIF keys that indicate a local diffusion pipeline (vs. a hosted

src/remove_ai_watermarks/noai/constants.py

@@ -85,6 +85,9 @@ C2PA_ISSUERS = {
     b"Microsoft": "Microsoft",
     b"OpenAI": "OpenAI",
     b"Truepic": "Truepic",
+    # Stability AI signs C2PA as "Stability AI" (cert org "Stability AI Ltd").
+    # Verified on a live Brand Studio (DreamStudio successor) output, 2026-05-24.
+    b"Stability AI": "Stability AI",
 }
 
 # C2PA issuers whose signed outputs also carry an invisible SynthID pixel