Stop double-counting a named forensic mark as SynthID provenance

A manifest that names its own forensic soft-binding algorithm carries
that vendor's mark; the generic watermark-action vendor-token inference
must not add a second, differently-attributed invisible watermark from
the same bytes. Microsoft Designer manifests triggered exactly that:
signed by Microsoft, watermarked by InvisMark, with the generation
agent named "Azure OpenAI ImageGen" - the OpenAI issuer token inside
that service name plus the InvisMark watermarked action satisfied the
OpenAI SynthID-evidence rule, and identify reported one forensic mark
as two paid pixel watermarks.

Three changes, one rule at every inference site (the verdict-scan
comment's own lesson: a rule that lives in only one copy is a rule the
others silently lack):

- c2pa.py structured path: SynthID evidence now scopes to the
  signer/generator identity strings only (signature issuer, claim
  generator), never the raw chain, and is suppressed entirely when a
  soft-binding algorithm is named.
- c2pa.py byte fallback and metadata.py synthid_source: suppressed when
  the scan names a soft-binding algorithm.
- identify.py verdict scan: same suppression.

Gemini and ChatGPT originals keep their provenance-asserted SynthID
strings; the Designer regression is pinned by
test_designer_synthid_suppression.py (agent name alone is not the
vendor's provenance, and a named soft binding suppresses the
inference).

src/remove_ai_watermarks/metadata.py

@@ -839,6 +839,13 @@ def synthid_source(image_path: Path, *, c2pa_info: dict[str, Any] | None = None)
     ai_source = b"trainedAlgorithmicMedia" in data or b"TrainedAlgorithmicMedia" in data
     if not (has_c2pa and ai_source):
         return None
+    from remove_ai_watermarks._internal.c2pa import soft_binding_vendors_in
+
+    # A scan that names its own forensic soft-binding algorithm carries that
+    # vendor's mark; the generic vendor-token inference must not add a second,
+    # differently-attributed invisible watermark from the same bytes.
+    if soft_binding_vendors_in(data):
+        return None
     matched = synthid_evidence_vendors_in(data)
     return ", ".join(matched) if matched else None