diff --git a/docs/port/DECISIONS.md b/docs/port/DECISIONS.md
index 475dce80..ab5c9099 100644
--- a/docs/port/DECISIONS.md
+++ b/docs/port/DECISIONS.md
@@ -16469,3 +16469,49 @@ stating because I did not have to assume which clock the record runs on.
 ⚠️ So the ask narrows to one number and gets a precise window: **a capture anywhere
 in t mod 120 ∈ [35, 50]**, where α holds at its peak of 80. Everything else about
 the plate is now settled on my side.
+
+### Pre-registered: what the plate capture must show, committed before it exists
+
+The Decoder's R2 note, and it is the right one to raise: α peaks at **80 of 255**,
+so an additive overlay at 31 % is a small signal and *"whatever residual you
+measure will be small in absolute terms whether or not the blend is right."*
+**So the expected magnitude goes in git before the capture does.**
+
+Rendering the same pose at peak α under both composites:
+
+| | value |
+|---|---|
+| highlight footprint at peak α | **25 015 px** (2.90 % of frame) |
+| mean \|additive − alpha-over\| **inside** it | **16.92 levels** |
+| max | 50 levels |
+| RMS over the **whole frame** | **3.746** |
+
+🔴 **And my first prediction was wrong, which is the useful part.** The two
+composites differ by exactly `α·bg`, and with mean background 94.78 that predicts
+**29.73** levels. Measured: **16.92**. The formula uses the *keyframe* α and
+ignores that **the sprite carries its own per-pixel alpha channel**, which
+modulates it — implied mean sprite α ≈ 145/255. Caught because I wrote the
+prediction down and it disagreed with the render, which is the whole point of
+writing it down.
+
+**The pre-registered test.** Additive is brighter than alpha-over at *every*
+footprint pixel, in one direction. So the discriminator is **regional sign over
+25 015 pixels, not per-pixel magnitude** — which matters because the port already
+carries global tone offsets of order 8.5 levels, and a per-pixel magnitude test
+would be swamped by them while a signed regional one is not.
+
+> **If the game composites this additively, then rendering it alpha-over must leave
+> the footprint systematically ≈17 levels DARKER than the capture relative to the
+> surrounding frame, and rendering it additive must leave no such regional bias.
+> A capture that shows neither bias refutes both, and points at the ramp or the
+> pose rather than the blend.**
+
+⚠️ **Feasibility, stated before asking anyone to spend a run:** 16.92 mean levels
+across 2.9 % of the frame against ~8.5-level systematic offsets is discriminable
+**only** as a signed regional statistic. As a whole-frame RMSE it is **3.746** —
+below the title's own ±5.56 capture-phase term, so `verify-capture`'s headline
+number could not settle this even at peak α.
+
+⚠️ Their gate `wait_plate_pulse.py` fires on a green-glyph count in [500, 2500] and
+has logged 740 and 1004, so the window may come free or may sit outside [35, 50]
+entirely. Their caveat, carried across rather than assumed away.