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/REFUTED.md

@@ -528,3 +528,13 @@ neighbourhood, not just the line.
   ≥77, against 6–8 predicted by the current reading and 80–102 by the shifted
   one. The `_eff` glows fit both and argue against neither.
   [`ui-keyframe-time-unit.md`](ui-keyframe-time-unit.md)
+* "an element with no held pose should be drawn as NOTHING rather than at a
+  guessed endpoint" → **mine, and refuted.** Suppressing every plateau-less
+  element and re-correlating against the live captures: title +0.9500 → +0.6839,
+  main menu +0.9460 → +0.9037, `EXTRAS` +0.9440 → +0.9094. Worse on all three.
+* "`rest()` guesses for 24.57 % of elements (3 807 of 15 493)" → **mine, and
+  overstated by 65 %.** The plateau test marks a **single-keyframe** element as
+  plateau-less because it has no adjacent pair — but its one pose is
+  unambiguously its rest. 1 502 of the 3 807 are those; the genuinely ambiguous
+  population is **2 305 (14.88 %)**.
+  [`ui-resting-pose.md`](structures/ui-resting-pose.md)