Record the qwen-zimage global denoise boundary against the Gemini oracle

A ladder on one native 2816x1536 Gemini original at seed 0, verified through the
Gemini app, puts the boundary between 0.08 and 0.10: 0.154, 0.12 and 0.10 read
clean, 0.08 reads SynthID FOUND. Fidelity rises monotonically all the way down,
so 0.10 buys +1.54 dB whole-image and +0.98 dB inside the face boxes over the
0.154 the profile ships for that size.

Recorded with the two constraints that stop it being acted on directly. It
brackets rather than calibrates - one image, one seed, and shipping the lowest
clean rung means shipping at the measured cliff edge. And the untested end is the
bottom, not the top: every Gemini oracle fixture is 2816x1536, so the Google-side
certification only ever covered 0.154, while the curve sends sub-1 MP images to
0.084-0.094. The resolution trend already recorded in this document says lower
processing resolution needs less strength, which is the shape the curve has, but
that is an inference and no small Gemini original has ever been through the
oracle. Downscaling is valid test material since SynthID survives it by design.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docs/synthid.md

@@ -544,6 +544,39 @@ detector. An explicit `--strength` always wins. If the watermark still survives
 large native Gemini beyond the capped-1536 validation), raise toward 0.35-0.40 (0.40
 visibly corrupts dense text), using the lowest value that reads clean on the oracle.
 
+**qwen-zimage global denoise, Gemini boundary bracketed (2026-08-02).** The profile does
+not use the vendor ladder above; `resolution_adaptive_denoise` maps megapixels onto
+roughly 0.084 (sub-0.3 MP) to 0.154 (>= 3.7 MP). A ladder on one native 2816x1536 Gemini
+original, seed 0, everything else at profile defaults, verified through the Gemini app:
+
+| global denoise | Gemini app | whole-image PSNR | face-box PSNR | edge IoU |
+|---|---|---|---|---|
+| 0.154 (profile top) | clean | 24.72 | 31.19 | 0.188 |
+| 0.12 | clean | 25.65 | 31.82 | 0.202 |
+| 0.10 | **clean** | 26.26 | 32.17 | 0.212 |
+| 0.08 | **SynthID FOUND** | 26.95 | 32.51 | 0.227 |
+
+So the boundary sits between 0.08 and 0.10 for this image, and the profile's shipped
+0.154 carries roughly half a rung more strength than that content needed. Fidelity rises
+monotonically all the way down - dropping to 0.10 buys **+1.54 dB whole-image and
++0.98 dB inside the face boxes** - which is exactly why the temptation is to move the
+ceiling, and exactly why one fixture is not enough to do it.
+
+Two constraints on reading this:
+
+- **It brackets, it does not calibrate.** One image, one seed. Shipping the lowest clean
+  rung means shipping at the measured cliff edge; another sample, seed, or content class
+  can sit on the other side of it. Note §5.2's flat-graphic hard cases were not in this
+  set at all.
+- **The bottom of the curve is the untested end, not the top.** Every Gemini oracle
+  fixture is 2816x1536, so the Google-side certification only ever covered 0.154, while
+  the curve sends sub-1 MP images to 0.084-0.094 - at or below the rung that failed here.
+  That is less alarming than it first looks, because the resolution trend recorded above
+  (line ~644) says lower processing resolution needs *less* strength, which is the shape
+  the curve already has. But it is an inference, not a measurement: no small Gemini
+  original has ever been through the oracle. Downscaling is safe test material - SynthID
+  survives it by design - so a downscaled original is a valid way to close that gap.
+
 ### 5.3 Test methodology
 
 - **GitHub-recompressed JPEGs from issue attachments are valid SynthID test