re(ui): a group's duration is in the data, its start time is not

Tested whether the splash timeline, played, reproduces the capture --
the last gap in the animation model. Half of it does.

Durations match. At 2 units/frame under the shifted reading, from the
235-frame draw capture of the developer splash:

  glows  drawn f94-115  (22 frames = 44 units)  declared ~0..45  = 45   97.8%
  logos  drawn f116-211 (96 frames = 192 units) declared 15..210 = 195  98.5%

Each element is on screen for its declared span to within 2%.

Starts do not. Every glow declares the same times 15,30,45 and every
logo the same 15,30,190,194,206,210, so on one clock they would overlap
almost entirely -- and they do not overlap at all. The glows run 94-115
and the logos 116-211, strictly sequential, the logos starting the frame
after the glows end. Fitting one origin needs f0 ~ 93.5 for gamearts_eff
and ~103 for gamearts, about 19 units apart, and aligning one throws the
other off by ~9 frames at both ends.

The obvious candidate is refuted. parse_placements reads each group
header as (element index, keyframe count) plus one undecoded LEAD-IN
WORD -- exactly where a per-group start offset would live. It is
0x00000000 for all seven elements, glows and logos alike.

Reach: not the keyframe times (identical within each family), not that
word (zero), not declaration order (which interleaves logos and glows
where the observed sequencing groups them), not the RATC child order.
What remains is that the sequencing is code-driven, which agrees with
what the boot-order work concluded independently.

For the port: a group says how long an element animates and what it does,
not when it starts relative to its neighbours. The observed order on the
developer splash -- both glows, then both logos -- is measured for one
screen, not a decoded rule, so the sequencing has to be authored.

METHOD: when a model reproduces durations but not positions, the missing
piece is an origin, not a rate.

docs/re/structures/ui-group-start-time.md

@@ -0,0 +1,62 @@
+# ❔ A group's DURATION is in the data; its START TIME is not
+
+**Status:** ✅ the durations are measured and match; ❔ the start offset is
+**undecodable from the placement data**, with the obvious candidate refuted.
+
+Found while testing whether the splash timeline, *played*, reproduces the
+capture sequence — the last gap in the animation model.
+
+## The durations are right
+
+From the 235-frame draw capture of the developer splash, at the settled
+2 units/frame, under the shifted time reading:
+
+| element | drawn on frames | = units | declared visible span | match |
+|---|---|---|---|---|
+| `palogo_gamearts_eff`, `palogo_seta_eff` | 94–115 (22) | 44 | ~0 … 45 = **45** | **97.8 %** |
+| `palogo_gamearts`, `palogo_seta` | 116–211 (96) | 192 | 15 … 210 = **195** | **98.5 %** |
+
+(The logos are `a = 0` until t=15, so they are *drawn* for 15…210; the glows fade
+in from their untimed first keyframe.) **Each element is on screen for its
+declared span, to within 2 %.**
+
+## 🔴 But they do not share a clock origin
+
+Every glow declares the same times — `15, 30, 45` — and every logo the same
+`15, 30, 190, 194, 206, 210`. On one clock they would overlap almost entirely:
+both families start at 15.
+
+**They do not overlap at all.** The glows run frames **94–115** and the logos
+**116–211** — strictly sequential, the logos starting the frame after the glows
+end. Fitting a single origin bears this out: `gamearts_eff` needs f₀ ≈ 93.5 and
+`gamearts` needs f₀ ≈ 103, about **19 units apart**, and the same edit that
+aligns one throws the other off by ~9 frames at both ends.
+
+## 🔴 The obvious candidate is refuted
+
+`parse_placements` reads each group header as `(element index, keyframe count)`
+followed by **one undecoded lead-in word**. That is exactly where a per-group
+start offset would live. It is **`0x00000000` for all seven elements** of the
+developer splash — glows and logos alike. It carries no offset.
+
+## ❔ So where does the start come from?
+
+Not from the keyframe times (identical across each family), not from the group
+header's spare word (zero), and not from declaration order (`eff0.prm`, gamearts,
+gamearts_eff, seta, seta_eff, anima, anima_eff — which interleaves logos and
+glows, while the observed sequencing groups them). The RATC child order does not
+match the observed order either.
+
+Reach: three places looked, all negative. What remains is that the sequencing is
+**driven by code**, consistent with what
+[boot-config-and-gamepart-registry](../boot-config-and-gamepart-registry.md)
+already concluded for the boot order — *the driver is code, not data*.
+
+## ⚠️ What a port needs to know
+
+A group tells you **how long** an element's animation runs and what it does; it
+does **not** tell you **when** that animation starts relative to other elements
+on the same screen. On the developer splash the observed order is: both glows
+together, then both logos together. That is a **measured** sequence for one
+screen, not a decoded rule — a port animating a screen has to author the
+sequencing, and should know it is authoring.