docs/re/METHOD.md

@@ -2389,3 +2389,30 @@ reads prose against a register, the port's checks read numbers against a tree, a
 a comment that contradicts the function under it is invisible to both. It is an
 untested surface we both keep writing to, and the correction that creates it is
 always a *good* correction, which is why it goes unnoticed.
+
+## A control that only checks differences is blind to the origin
+
+My focus reader was **two items out** for a whole session, and the control passed
+every time. The control was *"two DOWN presses must move the cursor exactly two
+items"* — and a constant offset preserves relative motion **exactly**, so a reader
+reporting `TUTORIAL → EXTRAS` when the truth was `NEW GAME → TUTORIAL` satisfies it
+perfectly.
+
+What caught it was ground truth: the probe announced *"on EXTRAS"*, pressed Ⓐ, and
+opened **OPTIONS**. I looked at the frame.
+
+The cause is worth naming because it is invisible in a diff: `menu_focus.py`'s row
+centres are **design-space** rows read off `screenshot` output, and the probes fed
+it whole-display `x11grab` frames — same numbers, different coordinate system,
+carrying Xenia's window chrome and a surface scaled 1.060.
+
+⚠️ **So: a differential control validates a differential claim only.** Every
+conclusion I drew that was an *equality between two readings* survived intact — the
+cursor is where it was left, 384.0 vs 385.5 — because a constant offset cancels.
+Every conclusion that was a *name* was wrong. Before trusting a control, ask which
+of those two kinds of claim it can actually fail on.
+
+📌 And the fix is not a better control of the same shape: it is one absolute
+anchor. `ring_row.py` now reports the ring's **measured row** and refuses to name
+an item when the row is not within half a step of a calibrated centre — refusing is
+the point, because a wrong name is what it exists to prevent.