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>

docs/supported-signals.md

@@ -172,9 +172,15 @@ documentation never mentions the seal. The
 default `qwen-zimage` profile clears Content Seal at the default
 resolution-adaptive strength (oracle-verified on 2.56 MP generations); measured
 strength boundaries are recorded in `data/contentseal/manifest.csv` and
-[module internals](module-internals.md#meta-content-seal-boundaries-for-qwen-zimage)
-(derived Meta floor 0.1 by the standard spread method, not shipped as a constant
-because no provenance signal can route Muse output onto a vendor cohort).
+[module internals](module-internals.md#meta-content-seal-boundaries-for-qwen-zimage).
+The derived Meta floor (0.1 by the standard spread method) ships as a measured
+cohort: auto mode routes a file whose only provenance is the standalone AI IPTC
+tag to it (the tag is not Meta-exclusive; other tag users ship no invisible
+watermark this profile targets, and Google/OpenAI/Microsoft C2PA evidence
+always wins first), and `invisible --vendor meta` (also `all` and `batch`, and
+`InvisibleOptions.vendor` in the API) names the cohort explicitly on stripped
+files. An explicit vendor implies the scrub runs: naming the cohort asserts the
+pixel watermark is present.
 The seal survives resizing, JPEG recompression, and metadata stripping; it dies
 to center crops of a third to a half, matching the Reuters 2026-07-11 finding
 that Meta's detector missed 55% of cropped Muse images.