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/port/HANDOFF.md

@@ -92,6 +92,20 @@ authored version can be deleted.
   [`ui-focus-and-effect-elements.md`](../re/structures/ui-focus-and-effect-elements.md)).
 * **Highlighted states pair by name** — `ptbtn01.rat` ↔ `ptbtn01f.rat`. 🟡 a
   naming convention that holds for all 54 real pairs, not a decoded field.
+* **`rest()` was wrong for elements with no exit animation — fixed 2026-08-28.**
+  A trailing run of identical keyframes was always treated as the exit and
+  excluded; on an element that has no exit it *is* the hold, and `rest()` fell
+  back to the element's **first** keyframe — off-position and transparent. Six
+  elements on the main menu were affected, including `ptframe1`/`ptframe2`, the
+  bright circuit bracket around the menu, which both the port's composite **and**
+  `sylpheed-cli screen render` were dropping.
+  The rule now: **a trailing run is the hold exactly when it is visible** (alpha
+  ≠ 0). ⚠️ Not the pose-equality test the report proposed — `pgptitle.rat`'s
+  trailing run also matches its last timed keyframe, and adopting that would erase
+  the word PAUSE. Oracle correlation over the bracket region improved
+  **0.9596 → 0.9748**; the PAUSE control is unchanged.
+  This also closes the old ❔ on `ptframe1`/`ptframe2` "resting at alpha 0 but the
+  capture shows the frame plainly".
 * **The resting pose is the hold**, not the first, last or longest-dwell keyframe;
   a keyframe is the **start of a ramp**.
   [`ui-resting-pose.md`](../re/structures/ui-resting-pose.md). ✅