re: rest() fixed -- a trailing keyframe run is the hold when it is

VISIBLE, not when its pose repeats

The port agent's report was right about the defect and about which
elements it hits, and its proposed condition does not survive the case
the exclusion was written for.

Their test -- the final untimed keyframe has the same pose as the last
timed one -- is also true of pgptitle.rat, whose trailing run is two
identical transparent frames. Adopting it would erase the word PAUSE
again, which a committed capture disproves. What separates the two is
alpha: an exit fades the element out so its last keyframe is transparent,
while an element with no exit ends on the pose you can see. So
rest_plateau now accepts a trailing run exactly when it is visible.

Verified against a capture rather than against another renderer, which is
the point their own report made: ptframe1's rest moves from (620,108)
t=16 to (440,108) t=62; the changed pixels are 10082 in a bounding box of
x 440-839, y 108-577, which is exactly the 400x470 at (440,108) they
predicted; and correlation against main-menu-oracle.png over that region
improves from 0.9596 to 0.9748. The PAUSE wordmark is unchanged across
all three pause builds.

This also closes a question ui-paint-order-key.md has carried for a
while, that ptframe1 and ptframe2 rest at alpha 0 while the capture shows
the menu frame plainly.

One trap cost most of this iteration and goes in METHOD: CARGO_TARGET_DIR
is redirected in this container, so ./target/debug/sylpheed-cli is hours
stale and every render I made against it was old code. Byte-identical
before and after reads as "no effect" when it actually means "you ran the
old binary".

The full disc-gated test suite was still executing when this was
committed; the verification above is artifact-based.

docs/re/structures/ui-resting-pose.md

@@ -185,5 +185,38 @@ This also explains a long-standing ❔ on
 `0x00ffffff` (alpha 0) and are therefore not drawn, but the capture shows the menu
 frame plainly."* Same two elements, same cause — now identified.
 
-❔ **Not yet fixed.** The change belongs in `ui_layout`'s `rest()`; it is a
-decoder change and has not been made.
+## ✅ Fixed 2026-08-28 — but the condition is the ALPHA, not the pose
+
+The report's proposed test — *"the final untimed keyframe has the same pose as the
+last timed one"* — **misfires**, and on the exact case the exclusion was written
+for. `pgptitle.rat`'s last two keyframes are also identical:
+
+```
+pgptitle.rat   kf4: fade=0x00ffffff pos=(220,69) t=30
+               kf5: fade=0x00ffffff pos=(220,69) t=None     <- same pose
+```
+
+Adopting it as stated would erase the word PAUSE again. What separates the two is
+**visibility**:
+
+| | trailing run | alpha | is it the hold? |
+|---|---|---|---|
+| `ptframe1`/`ptframe2` (main menu) | 3 × `0xffffffff` at (440,108) | `0xff` | **yes** — no exit animation |
+| `pgptitle` (pause menu) | 2 × `0x00ffffff` | `0x00` | no — it is the fade-out |
+
+An exit fades the element out, so its last keyframe is transparent; an element
+with no exit ends on the pose you can see. **So a trailing run is the hold exactly
+when it is visible**, and that is what `rest_plateau` now tests.
+
+### Verified against a capture, not against another renderer
+
+| check | result |
+|---|---|
+| `ptframe1` rest | `(620,108) t=16` → **`(440,108) t=62`** |
+| bracket region draws | mean 61.74 → **62.79** |
+| **oracle correlation** over that region vs [`main-menu-oracle.png`](../captures/main-menu-oracle.png) | **0.9596 → 0.9748** |
+| pixels changed, whole frame | 10 082, bounding box **x 440–839, y 108–577** — exactly the 400 × 470 at (440,108) the report predicted |
+| **regression control**: PAUSE wordmark, 3 pause builds | **unchanged** (2833 / 2858 / 2833 bright px) |
+
+🟡 The full disc-gated `build-reborn test` run was **still executing when this was
+committed**; the verification above is artifact-based.