re(ui): why rest_plateau is right -- and last is right only for a transient

The shifted keyframe-time reading looked like it implied something
simple: the final pose is reached at a definite time and nothing follows,
so rest should just be the last keyframe and the plateau heuristic could
go. Tested by applying it to EVERY element:

  title              +0.9500 -> +0.6819   -0.2681
  main menu          +0.9460 -> +0.6416   -0.3044
  EXTRAS             +0.9440 -> +0.5745   -0.3695
  publisher splash   +0.9600 -> blank (zero variance, corr undefined)
  developer splash   +0.9643 -> blank

Refuted, and the failure supplies the model. A group is entry -> hold ->
exit, and the exit is the screen's DISMISSAL. While a screen is displayed
it has not reached its last keyframe; it is sitting at the hold. So
rest_plateau is the correct primary rule, and the last keyframe is the
post-exit state -- correct only once the screen is gone, which is why
applying it everywhere blanks the splashes.

This does not contradict the shifted reading. That reading says when each
pose is reached; it says nothing about the group being played to
completion while the screen is still up. The step between them was mine.

And it explains why last wins for the two plateau-less elements: an
element with no hold is a transient, it flashes and is over, and at any
settled moment it is gone -- which is its last keyframe. The draw capture
says the same independently: on the developer splash the _eff glows draw
on frames 94-115 and the logos on 116-211, so the glows are already
finished when the logos are up.

Three independent observables -- animation timing, static composites, and
the per-frame draw log -- now agree on one rule: plateau where there is
one, last keyframe where there is not.

METHOD: a blank render is a NaN correlation, not a low score, and that
NaN was the strongest form of the result; and when a model predicts
something the measurement refuses, suspect the step you supplied between
them.

docs/re/METHOD.md

@@ -774,3 +774,16 @@ agent's loop prompt, i.e. nowhere durable. See [`README.md`](README.md) for the
   which felt like an artefact. Measured, it is the better rule on both screens
   where it applies. Aesthetic expectations about how authored data "should" look
   are worth stating as hypotheses and worth nothing as verdicts.
+* **A blank render is a correlation of NaN, not a low score — notice which.**
+  Applying "rest = last keyframe" to every element made both splashes render with
+  zero variance, so the correlation was undefined rather than poor. Reading the
+  NaN as "the tool failed" would have hidden the result; it *was* the result, and
+  the strongest form of it. Check for degenerate output before treating a missing
+  number as a broken measurement.
+* **When a model predicts something and the measurement refuses, the model is
+  usually incomplete rather than wrong.** The shifted time reading says when each
+  pose is reached, which looked like it implied the final pose is the resting
+  one. It does not: it says nothing about whether the group is *played to
+  completion* while the screen is still up. The exit is the dismissal, so a
+  displayed screen never reaches its last keyframe. Both claims survive; what was
+  wrong was the step between them that I supplied.