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/METHOD.md

@@ -396,3 +396,15 @@ agent's loop prompt, i.e. nowhere durable. See [`README.md`](README.md) for the
   the plateau the first path already caught. So the fallback is guaranteed to be
   guessing every time it is reached. When a heuristic's justification is the same
   condition that routes around it, it has no justification.
+* **Siblings adjudicate a rule that a single element cannot.** Deciding what
+  `rest()` should return for an element with no held pose looked like it needed a
+  capture we do not have. It did not: the developer splash carries three glows
+  with identical structure and identical times, differing in one alpha byte, and
+  any rule that makes one of them behave completely differently from the other
+  two is producing an artefact. Parallel constructions in the same bundle are a
+  free control — look for them before concluding a question needs new measurement.
+* **Backgrounding with `&` in a compound command loses the working directory.**
+  `cd X && cmd &` followed by more work in the same call left the shell reset, and
+  a heredoc two lines later wrote nowhere — while the `echo` after it still
+  printed "written". A success message after a failed redirect is not a success.
+  Use `(setsid nohup … &)` and keep the file-writing in its own call.