Read the InvisMark soft-binding algorithm and its signed watermark id

Reachable c2pa.soft-binding assertions now surface their exact alg and the
bounded printable block value next to the normalized vendor label; a value
without its algorithm stays hidden because it cannot be attributed.
com.microsoft.invismark.1 uses that value as the pixel-watermark identifier
in Microsoft Paint output, so identify names it, metadata carries it, and an
InvisMark soft binding keeps the invisible-removal gate fail-safe even after
C2PA asset binding goes invalid. Content fingerprints still do not trigger
pixel regeneration. Removal is verified against Microsoft's Content
Provenance Detection API, which reports Watermark and C2PA separately; the
protocol and the pixel-identical control requirement are documented.

Implemented in a parallel session; verified, gated, and committed by pi.

pre-commit: 1) maintain.sh - exit 1 on the known uv-secure lightning PYSEC-2026-3624 triage (no fix available, unchanged from 0.29.0); ruff, pyright src/, and 1391 tests passed separately; 2) /simplify - single-pass, clean; 3) docs sync - five docs updated by the author session, no remaining references found; 4) CLAUDE.md - invariants recorded in module-internals, no change needed

docs/verification-plan.md

@@ -186,10 +186,14 @@ report recall from the detector-sampled set.
 
 ## Tier D -- external oracles (manual, not automatable here)
 
-SynthID removal cannot be verified locally by design -- no public decoder exists. Each
-vendor has its own oracle and it covers only that vendor's content: `openai.com/verify` for
-OpenAI (more accessible, the automation candidate), the Gemini app for Google (manual,
-rate-limited). A quiet metadata proxy is **not** proof the pixel watermark is gone.
+Proprietary watermark removal cannot be verified locally by design -- no public decoder
+exists. Each vendor has its own oracle and it covers only that vendor's content:
+`openai.com/verify` for OpenAI, the Gemini app for Google, and Microsoft's
+[Content Provenance Detection API](https://learn.microsoft.com/en-us/azure/ai-services/content-safety/how-to/how-to-provenance-detection)
+for InvisMark. The Microsoft API reports `Watermark` and `C2PA` separately. It therefore
+needs a pixel-identical metadata-stripped control: the control must lose `C2PA` while
+remaining `Watermark`-positive before a candidate's negative result can be attributed to
+pixel regeneration. A quiet metadata proxy is **not** proof the pixel watermark is gone.
 
 Scope honestly: this tier certifies strength floors on a handful of images per vendor, and
 that is all it can do. See `docs/synthid.md`.