Ship the measured Meta Content Seal cohort with auto routing and --vendor override

Full Meta Muse Image support in the invisible-removal path:

- QWEN_ZIMAGE_META_STRENGTH = 0.1: derived by the standard
  worst-boundary-plus-cross-source-spread method over five oracle-bracketed
  generations (data/contentseal/manifest.csv)
- Auto mode: vendor_for_strength routes a file whose only provenance is the
  standalone AI IPTC trainedAlgorithmicMedia tag onto the meta cohort; C2PA
  issuers win first, so Google/OpenAI/Microsoft routing is unchanged. Muse
  WebP outputs place the XMP in a tail chunk, so the scan uses the shared
  chunk-aware metadata.scan_head rather than a plain head read
- Explicit override: --vendor on invisible/all/batch and
  InvisibleOptions.vendor name the cohort on stripped files; naming a cohort
  asserts the watermark is present, so the no-signal gate treats it like
  --force at both the CLI and API seams
- sdxl-zimage has no measured Meta rung: an explicit meta vendor falls to
  the conservative unknown 0.25 rather than inventing one
- identify emits a Content Seal caveat pointing at the removal path
- The legacy visible 'Imagined with AI' mark stays unregistered: a dedicated
  sample hunt (newsroom mockups, community posts, press screenshots, dead
  imagine.meta.com, broken Wayback captures) found no pixel-verifiable
  capture, and the registry rule forbids encoding a corner without one.
  erase --region remains its removal path; outcome recorded in the landscape

Co-Authored-By: Claude Fable 4.5 <noreply@anthropic.com>

src/remove_ai_watermarks/identify.py

@@ -135,6 +135,11 @@ _C2PA_INVALID_CAVEAT = (
     "are retained only as removal hints, not as verified provenance."
 )
 _IPTC_ONLY_CAVEAT = "The IPTC 'Made with AI' tag flags AI provenance but does not identify the specific platform."
+_CONTENT_SEAL_CAVEAT = (
+    "Meta Muse Image outputs carry the invisible Content Seal pixel watermark, which has no "
+    "local decoder; `invisible` removes it (auto when this tag is present, or `--vendor meta` "
+    "on stripped files) and meta.ai/identification verifies it."
+)
 _INVISIBLE_WM_CAVEAT = (
     "The open invisible watermark is fragile: it does not survive JPEG re-encoding "
     "or resizing, so it confirms origin only on a pristine (un-re-encoded) file."
@@ -1318,6 +1323,7 @@ def _identify_from_evidence(
         signals.append(Signal("iptc", "digitalSourceType (Made with AI)", "high"))
         watermarks.append("IPTC digitalSourceType (Made with AI)")
         caveats.append(_IPTC_ONLY_CAVEAT)
+        caveats.append(_CONTENT_SEAL_CAVEAT)
         if platform is None:
             # Apple Photos Clean Up (Apple Intelligence object removal) marks
             # the edit with photoshop:Credit / IPTC "Apple Photos Clean Up"