re(ui): a static composite is only meaningful for a screen that settles

The model's sharpest prediction, tested with its control. The draw log
says that on the developer splash the _eff glows are drawn on frames
94-115 and the logos on 116-211, so at the moment the reference capture
was taken EVERY glow is already finished -- including the two that have
plateaus and which rest_plateau therefore renders visible. Suppressing
them should help on the splashes and hurt where a screen genuinely
settles.

  publisher splash   +0.9604 -> +0.9982   +0.0377
  developer splash   +0.9659 -> +0.9980   +0.0321
  title    (control) +0.9500 -> +0.9480   -0.0020
  main menu(control) +0.9460 -> +0.8544   -0.0916
  EXTRAS   (control) +0.9440 -> +0.8370   -0.1070

Both splashes jump to about 0.998; all three persistent screens get
worse. The control is what makes this a finding rather than a
coincidence: the same edit helps exactly where the model says it should
and hurts exactly where it says it should not.

So rest_plateau is not over-drawing in general -- it over-draws on
TRANSIENT screens. A plateau mid-animation means the element is held at
that point in the timeline, not that it is on screen once the screen has
settled. Where a screen settles, the held pose IS the settled pose and
the rule is measurably right.

And that answers the question left open several iterations ago -- what
"rest" means for a transient element. It does not mean anything: the
splashes never rest. A static composite of them can match a chosen frame,
and about 0.998 is what these captures' frame is worth, but the format
does not answer a question the screen never poses.

For the port: play the timeline for the two splashes, which the settled
keyframe timing now supports, and composite statically for title, main
menu and EXTRAS.

METHOD: an edit that improves one set of cases is only interesting once
you have shown it damages the cases where it should.

crates/sylpheed-formats/examples/plateauless_suppression.rs

@@ -25,7 +25,15 @@ fn main() {
         let bytes = ar.read(&entries[idx]).expect("read");
         let Some(build) = ui_layout::parse_build(&bytes) else { continue };
         // plateau-less = rest() had to guess: no two adjacent keyframes share a pose
+        // Default: suppress plateau-less elements. With SUPPRESS_SUBSTR set,
+        // suppress every element whose NAME contains it instead — used to test
+        // the entry→hold→exit model's prediction that the splash glows are all
+        // finished by the moment the logos are up.
+        let by_name = std::env::var("SUPPRESS_SUBSTR").ok();
         let mask: Vec<bool> = build.elements.iter().map(|e| {
+            if let Some(sub) = &by_name {
+                return !e.name.to_lowercase().contains(sub.as_str());
+            }
             let k = &e.keyframes;
             (0..k.len().saturating_sub(1)).any(|i| {
                 k[i].fade == k[i+1].fade && k[i].scale_x == k[i+1].scale_x