docs: the splash .prm measured -- and its paint order is a sequence, not depth

ui-prm-primitives recorded that where a .prm paints on a screen without a
measured order is unsolved. For the developer splash it is now measured.

Every frame opens with two full-screen draws before any sprite. The
second is untextured in all 212 frames with a constant vertex colour of
FF000000 -- opaque black -- matching palogo_eff0.prm's declaration
exactly: kind 0x10, pivot (640,360) -> 1280x720, one keyframe, a = 255.
So the splash backdrop is an opaque black full-screen quad from the
bundle itself, painted behind every sprite, which is why a splash render
needs --black rather than the default backdrop.

Not a general rule, and said so: the measured main-menu order puts
pteff02.prm at position 4 and pteff00.prm LAST, the latter being the
screen-transition fade.

And a correction to an existing row. measured_paint_order returns
[0, 2, 4, 6, 1, 3, 5] for the splash, described as "the .prm, then all
three glows, then the three logos". But the glows and the logos never
appear in the same frame -- 0 overlapping frames in 235 -- and two
elements that never co-occur have no observable relative depth. Between
those halves the vector records the order they were SEEN IN, not a
front-to-back relationship.

That does not make the render wrong, and element 0 is a real depth
observation since the .prm co-occurs with everything. But the type of the
claim matters: reading the vector as depth invites compositing all seven
elements at once, which is exactly what does not reproduce the screen.

METHOD: two things that never co-occur have no observable relative order;
when recording an order, note which pairs actually appeared together.

docs/port/HANDOFF.md

@@ -386,6 +386,21 @@ authored version can be deleted.
   that settles, and it is not a timeline.
   [`structures/ui-group-start-time.md`](../re/structures/ui-group-start-time.md)
 
+* ✅ **The splash's backdrop is an opaque black `.prm` from the bundle, painted
+  first.** Measured: every frame opens with the clear and then an **untextured
+  1280×720 quad, vertex colour `FF000000` in all 212 frames** — matching
+  `palogo_eff0.prm`'s declaration (`kind 0x10`, pivot 640×360, one keyframe,
+  `a=255`). That is why a splash render needs `--black` rather than the default
+  backdrop. ⚠️ Not a general rule: the main menu's measured order puts one `.prm`
+  at position 4 and the other **last** (the transition fade).
+  ⚠️ **And the splash's recorded paint order `[0,2,4,6,1,3,5]` is a temporal
+  sequence, not a depth order.** Its glows and logos never share a frame, so
+  their relative depth was never observable — the vector records the order they
+  were *seen in*. Element 0 (the `.prm`) is a genuine depth observation because it
+  co-occurs with everything. **Do not read the rest as depth and composite all
+  seven at once** — that is precisely what fails to reproduce the screen.
+  [`structures/ui-prm-primitives.md`](../re/structures/ui-prm-primitives.md)
+
 * **Menu order is geometric.** Buttons sorted top-to-bottom by resting Y. This is
   ✅ correct for a vertical menu and is **not** a decoded neighbour graph — the
   disc's real navigation structure is unknown, and `opt ` is *not* a focus link