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/identify.py

@@ -1273,7 +1273,19 @@ def _identify_from_evidence(
     # reusing the derived `has_c2pa` / `source_kind` above, which are broader:
     # the file path's answer must not move.
     trained_source = b"trainedAlgorithmicMedia" in head or b"TrainedAlgorithmicMedia" in head
-    if not synthid and trained_source and c2pa_marker_in(head) and (vendors := synthid_evidence_vendors_in(region)):
+    # Same suppression as every other inference site: bytes that name their own
+    # forensic soft-binding algorithm carry that vendor's mark, and the generic
+    # vendor-token inference must not add a second, differently-attributed
+    # invisible watermark (Microsoft Designer: "Azure OpenAI ImageGen" agent +
+    # the InvisMark watermarked action read as "SynthID per OpenAI").
+
+    if (
+        not synthid
+        and trained_source
+        and c2pa_marker_in(head)
+        and not soft_binding_vendors_in(region)
+        and (vendors := synthid_evidence_vendors_in(region))
+    ):
         synthid = synthid_verdict(", ".join(vendors))
     if synthid:
         watermarks.append(