re(ui): the rest fallback fires on 2 elements, and "last keyframe" wins there

Scored candidate rest-pose rules by rendering and correlating instead of
arguing, and both results correct something I had published.

First, the exposure. The guessing fallback is reached only by an element
that is plateau-less AND multi-keyframe -- a single-keyframe element
short-circuits at `match len { 1 => first }`. Per screen:

  title (4)               24 elements   2 plateau-less   0 reach the fallback
  main menu (5)           16            5                0
  EXTRAS (6)              18            5                0
  publisher splash (10)    3            2                1
  developer splash (11)    7            2                1

So on the three screens the port cares most about, rest() never guesses.
That is why three different rules render builds 4/5/6 to identical
correlations -- the code is unreachable there, which I nearly read as
"the choice does not matter".

Second, where it does fire, the last keyframe is markedly better:

  publisher splash   dwell +0.9600   last +0.9982   maxalpha +0.9600
  developer splash   dwell +0.9643   last +0.9758   maxalpha +0.9643

That refutes my own earlier refutation. I had killed the last-keyframe
rule by arguing it makes palogo_anima_eff invisible while its two
siblings stay lit, which looked like an artefact. The capture says
otherwise: making it invisible is what improves the match. The sibling
symmetry was my expectation, not evidence.

Caveat kept in front: both captures are single frames of a transient
animation, so this fixes which pose matches THOSE frames, not which is
canonically at rest. Default unchanged -- better on both screens where it
fires and identical on the other three, but it would move 2 305 elements
disc-wide on two measurements. Reachable via SYLPHEED_REST_RULE=last.

Also confirmed: all 195 zero-scale rest poses are inside the corrected
2 305 ambiguous population; none is a single-keyframe element.

METHOD: score a rule where it can differ, or you measure nothing; and an
argument from symmetry is a prediction, not a refutation.

docs/re/REFUTED.md

@@ -538,3 +538,13 @@ neighbourhood, not just the line.
   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)
+* "`rest()` for a plateau-less element should be the last keyframe → refuted by
+  the sibling argument" → **that refutation is itself refuted, this time by
+  measurement.** Rendering under the rule and correlating against the live
+  captures: publisher splash +0.9600 → **+0.9982**, developer splash +0.9643 →
+  **+0.9758**. Making `palogo_anima_eff` invisible *improves* the match; the
+  sibling symmetry was my expectation, not evidence.
+* "the port's exposure to the rest-guessing defect is 14 elements" → **two.** The
+  fallback needs an element to be plateau-less **and** multi-keyframe; title,
+  main menu and `EXTRAS` reach it **zero** times, the two splashes once each.
+  [`ui-resting-pose.md`](structures/ui-resting-pose.md)