re(ui): the rest fallback fires on 2 elements, and "last keyframe" wins there

Scored candidate rest-pose rules by rendering and correlating instead of
arguing, and both results correct something I had published.

First, the exposure. The guessing fallback is reached only by an element
that is plateau-less AND multi-keyframe -- a single-keyframe element
short-circuits at `match len { 1 => first }`. Per screen:

  title (4)               24 elements   2 plateau-less   0 reach the fallback
  main menu (5)           16            5                0
  EXTRAS (6)              18            5                0
  publisher splash (10)    3            2                1
  developer splash (11)    7            2                1

So on the three screens the port cares most about, rest() never guesses.
That is why three different rules render builds 4/5/6 to identical
correlations -- the code is unreachable there, which I nearly read as
"the choice does not matter".

Second, where it does fire, the last keyframe is markedly better:

  publisher splash   dwell +0.9600   last +0.9982   maxalpha +0.9600
  developer splash   dwell +0.9643   last +0.9758   maxalpha +0.9643

That refutes my own earlier refutation. I had killed the last-keyframe
rule by arguing it makes palogo_anima_eff invisible while its two
siblings stay lit, which looked like an artefact. The capture says
otherwise: making it invisible is what improves the match. The sibling
symmetry was my expectation, not evidence.

Caveat kept in front: both captures are single frames of a transient
animation, so this fixes which pose matches THOSE frames, not which is
canonically at rest. Default unchanged -- better on both screens where it
fires and identical on the other three, but it would move 2 305 elements
disc-wide on two measurements. Reachable via SYLPHEED_REST_RULE=last.

Also confirmed: all 195 zero-scale rest poses are inside the corrected
2 305 ambiguous population; none is a single-keyframe element.

METHOD: score a rule where it can differ, or you measure nothing; and an
argument from symmetry is a prediction, not a refutation.

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

@@ -431,3 +431,63 @@ The same flaw is in the census this page published:
 
 So the guessed-rest population is **2 305, not 3 807** — the published figure
 overstated it by **65 %**. The defect is real and smaller than reported.
+
+---
+
+## ✅ Where the fallback actually fires — and "last keyframe" wins there
+
+**2026-08-29.** Two things fell out of scoring rules by rendering instead of
+arguing about them.
+
+### The port's exposure is TWO elements, not fourteen
+
+The guessing fallback is reached only by an element that is plateau-less **and**
+has more than one keyframe — a single-keyframe element short-circuits at
+`match len { 1 => first }`. Per screen:
+
+| screen | elements | plateau-less | **reach the fallback** |
+|---|---|---|---|
+| title (4) | 24 | 2 | **0** |
+| main menu (5) | 16 | 5 | **0** |
+| `EXTRAS` (6) | 18 | 5 | **0** |
+| publisher splash (10) | 3 | 2 | **1** |
+| developer splash (11) | 7 | 2 | **1** |
+
+On the three screens a port cares most about, **`rest()` never guesses at all**.
+That is why three different fallback rules render builds 4, 5 and 6 to *identical*
+correlations (+0.9500 / +0.9460 / +0.9440 for every rule) — the code is
+unreachable there.
+
+### On the two splashes, the last keyframe is markedly better
+
+Rendering under each rule (`SYLPHEED_REST_RULE=last|maxalpha`, default off) and
+correlating against the live captures:
+
+| splash | dwell (current) | **last** | maxalpha |
+|---|---|---|---|
+| publisher | +0.9600 | **+0.9982** | +0.9600 |
+| developer | +0.9643 | **+0.9758** | +0.9643 |
+
+`maxalpha` is indistinguishable from the current rule; **`last` gains +0.038 and
++0.012**, and 0.9982 is very close to exact.
+
+### 🔴 This refutes my own refutation
+
+I had killed "rest = the last keyframe" by argument: on the developer splash it
+makes `palogo_anima_eff` invisible while its two siblings stay lit, which looked
+like an artefact. **The capture says otherwise** — making it invisible is what
+improves the match. The sibling symmetry was my expectation, not evidence.
+
+⚠️ **The caveat that keeps this honest:** both captures are *single frames of a
+transient animation*. They fix which pose matches **these reference frames**, not
+which pose is canonically "at rest" — a grab a second earlier would show the
+glows. What the measurement supports is: for reproducing the committed captures,
+use the last keyframe.
+
+🟡 **Default unchanged.** The rule is better on both screens where it fires and
+provably identical on the other three, but it would alter the rest pose of
+**2 305** elements disc-wide and two of them have been measured. Reachable via
+`SYLPHEED_REST_RULE=last` for anyone who wants to take that further.
+
+✅ Unaffected by the earlier correction: all **195** zero-scale rest poses are
+inside the genuinely-ambiguous 2 305 — none is a single-keyframe element.