re(ui): size the rest() guess disc-wide, and refute my own proposed fix

Follows up the defect found last iteration: rest()'s dwell fallback is
guessing whenever it runs. Two things were open -- how big it is, and
whether "rest = the last keyframe" is the fix. Both are now answered, and
the second is answered no.

plateau_census.py walks the placement regions directly instead of going
through `screen info --geometry`, which decodes every texture and cannot
do a disc-wide pass in reasonable time. Its control reproduces GP_TITLE
build 7's three fallback elements and names ptlogo_eff3.t32 among them
before counting anything.

  elements with a keyframe group      15 493
  no plateau -> rest pose is guessed   3 807  (24.57 %)
    ... current rule returns invisible 1 711  (44.9 %)
    ... current rule returns scale=0     195  ( 5.1 %)
  the two candidate rules agree        1 911  (50.2 %)

195 elements get a rest pose with scale 0%, which is not a pose. And
disc-wide the choice of rule is not cosmetic: the candidates agree half
the time.

But the port's exposure is one element. Across main menu, EXTRAS, title
and the developer splash, 14 elements are plateau-less and the two rules
agree on 13. The single disagreement is palogo_anima_eff.t32.

And "last keyframe" loses there, on a control that needed no new capture:
the splash carries three sibling glows with identical structure and
identical times --

  palogo_gamearts_eff  15:a=0 30:a=255 45:a=255 -:a=0  plateau -> visible
  palogo_seta_eff      15:a=0 30:a=255 45:a=255 -:a=0  plateau -> visible
  palogo_anima_eff     15:a=0 30:a=255 45:a=212 -:a=0  no plateau

-- differing in one byte. "Last keyframe" makes anima alone invisible
while its two siblings stay lit. The capture agrees weakly: box-mean
ratios capture/render are gamearts 0.717, seta 0.723, anima 0.772, and a
glow we drew that the game does not would put anima below its siblings,
not above.

So the defect is measured and the fix is still undecided. Nothing in the
decoder changed.

docs/re/REFUTED.md

@@ -402,3 +402,9 @@ neighbourhood, not just the line.
   poses; an endpoint is only held when the two poses are equal, which is a
   plateau, which the earlier path already returned for. Every element that
   reaches the fallback has a guessed rest pose.
+* "`rest()` for a plateau-less element should be the **last keyframe**" → **mine,
+  and refuted.** The developer splash's three sibling glows are structurally
+  identical and differ by one byte (`a=212` vs `a=255` at `t=45`); that rule
+  makes `palogo_anima_eff` alone invisible while `gamearts_eff` and `seta_eff`
+  stay lit. Capture box-mean ratios (0.717 / 0.723 / **0.772**) go the other way
+  too. [`structures/ui-resting-pose.md`](structures/ui-resting-pose.md)