Turn the verified-text fidelity anchor off by default

The whole-frame 15% Qwen-VAE blend returned detector-visible OpenAI
SynthID on poster-scale manifests through the engine text-manifest
path (official Content Provenance API, 2026-08-19: restored detected
x6 with the anchor, clean x6 without it; base outputs clean x6;
pixel-identical stripped controls detected, proving the pixel channel).
Add fidelity_anchor=False to remove_watermark and InvisibleOptions and
--fidelity-anchor on the CLI to reproduce the 0.27.0 research
behavior. Text-box MAE cost of the new default is under one point on
all three fixtures (11.60->11.72, 7.79->7.86, 7.57->8.13).

docs/python-api.md

@@ -614,6 +614,13 @@ postprocessing combinations that were not evaluated. `InvisibleOptions` exposes
 same field for `remove_all`; after a visible-stage edit, the manifest must be built
 against the staged pixels rather than the pristine source.
 
+Since 0.27.1 the mode's global 15% Qwen-VAE fidelity-anchor blend is **off by
+default** (`fidelity_anchor=False`): that whole-frame blend was measured to
+return detector-visible OpenAI SynthID on poster-scale manifests (official
+Content Provenance API, 2026-08-19 - detected x6 with the anchor, clean x6
+without it, controls and base outputs validated in the same sessions). Pass
+`fidelity_anchor=True` to reproduce the 0.27.0 research behavior.
+
 `remove_watermark` takes strength, seed, tiling, resolution, and postprocessing
 controls. It takes no model id, step count or guidance scale, and neither does the
 constructor: each profile pins its model stack, its per-stage schedule and CFG