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/port/HANDOFF.md

@@ -398,8 +398,11 @@ authored version can be deleted.
   pose k **to** pose k+1; neither is held unless they are equal — which is a
   plateau, and the plateau path has already returned by then. **So any element
   with no two adjacent identical poses has a guessed rest pose**, in our renderer
-  and in anything built from it. Measured disc-wide: **3 807 of 15 493 elements
-  (24.57 %)**, of which **195 get a degenerate `scale = 0 %` pose**, and the two
+  and in anything built from it. Measured disc-wide: **2 305 of 15 493 elements
+  (14.88 %)** — ⚠️ **corrected 2026-08-29 down from a published 3 807 (24.57 %)**,
+  which counted 1 502 *single-keyframe* elements as guesses; those have one pose
+  and it is unambiguously their rest. Of the real 2 305, **195 get a degenerate
+  `scale = 0 %` pose**, and the two
   candidate rules agree only **50.2 %** of the time.
   ✅ **This does not block you.** On the five screens the port needs, 14 elements
   are affected and the candidate rules **agree on 13**. The single disagreement