re(ui): refute my own fix for rest(), and correct the defect rate by 65%

Two corrections from one experiment.

A keyframe group is entry -> hold -> exit, and the exit ends invisible:
on the five port screens the final keyframe is invisible for 21/24
(title), 8/16 (main menu), 12/18 (EXTRAS), 2/3 and 6/7 (splashes). So the
screen as seen is the HOLD, which is why rest_plateau is the right
primary rule and why "rest = last keyframe" would empty every screen.

That suggested a fix: an element with no hold has no representative pose,
so draw nothing rather than guess an endpoint. Tested through compose's
visible mask and correlated against the live captures:

  title       +0.9500 -> +0.6839   -0.2661
  main menu   +0.9460 -> +0.9037   -0.0423
  EXTRAS      +0.9440 -> +0.9094   -0.0346

Refuted on all three, and the reason invalidates a number I published. An
element with a SINGLE keyframe has no adjacent pair, so the plateau test
marks it plateau-less -- but its one pose is unambiguously its rest.
Suppressing those removes backgrounds and full-screen layers, which is
the title's -0.27.

  no plateau (as published)      3 807  (24.57 %)
    ... single-keyframe          1 502  trivially at rest, not a guess
    genuinely ambiguous          2 305  (14.88 %)

So rest() guesses for 2 305 elements, not 3 807 -- the figure I gave the
port overstated the defect by 65%. Corrected in HANDOFF and the page.

METHOD: a predicate over adjacent PAIRS silently misclassifies a
one-element list; and acting on a claim is a better test of it than
re-reading it -- this flaw survived a census, a write-up and a handoff
row, and died the moment the rule was used to change a rendering.

docs/re/METHOD.md

@@ -750,3 +750,15 @@ agent's loop prompt, i.e. nowhere durable. See [`README.md`](README.md) for the
   had three discriminating measurements; pose-selection had a heuristic guessing.
   When a conclusion is blocked, check whether the blocker is actually evidence
   about the same thing.
+* **A predicate over adjacent PAIRS silently misclassifies a one-element list.**
+  "Has a plateau" was implemented as *any two adjacent keyframes share a pose* —
+  which is false for a single-keyframe element, so 1 502 static elements were
+  counted as having a *guessed* rest pose and the published defect rate was 65 %
+  too high. The error only surfaced when acting on it: suppressing those elements
+  dropped the title's correlation by 0.27, because they include the backgrounds.
+  Whenever a rule quantifies over pairs, ask what it says about a list of one.
+* **Acting on a claim is a better test of it than re-reading it.** The
+  single-keyframe flaw survived a disc-wide census, a write-up and a handoff row.
+  It died the moment the rule was used to change a rendering, because the result
+  was visibly worse. If a measurement implies an action, take the action on
+  something you can score.