re(ui): premultiplied alpha refuted for bit 0x02; parking the field

A per-sprite premultiplied-vs-straight-alpha flag would matter a lot to a
port and has a sharp static signature: premultiplied means RGB <= A
everywhere. Over the 170 decoded GP_TITLE textures that pair to a flag
word:

  bit SET    n= 61   mean %(RGB>A) 55.52   median 52.52
  bit clear  n=109   mean %(RGB>A) 33.66   median 30.17

Premultiplied requires ~0% for the flagged group. Both groups are far
from it and the flagged group violates MORE -- the opposite of the
hypothesis. Refuted.

What remains is a weak association: flagged sprites carry more
bright-RGB/low-alpha pixels, which is what glow art looks like. But the
best single threshold classifies 76.5% against a 64.1% base rate -- a
12-point lift with badly overlapping distributions. A tendency, not a
rule, and reported with its base rate so it cannot read as more.

Noted for whoever returns: "0x02 selects an additive blend" was refuted
by blending those sprites additively and finding every measure worse
against the capture -- but that ran through a title render since fixed
twice (rest_plateau, and the 8AX background the composer drops). The
refutation may well stand; it was measured through a renderer with known
other errors, so it is worth one re-run if blit ever gains additive
blending.

Parking the field. Four candidate meanings are dead -- additive blend,
eff name in both directions, transient element, premultiplied alpha --
none produced a positive account, and the bit blocks nothing: the port's
screens composite at 0.947 correlation against a capture without it. The
negative space and the sound attribution method (child order, not size)
are written down so a later attempt starts here.

METHOD: report a classifier's lift over its base rate; and park a field
after N failed hypotheses, saying what was eliminated.

docs/re/REFUTED.md

@@ -511,3 +511,8 @@ neighbourhood, not just the line.
   fails on **2 657 of 4 995** bit-set ones. `P(eff|set) = 0.468` against
   `P(eff|clear) = 0.144` — an association, not an implication.
   [`ui-paint-order-key.md`](structures/ui-paint-order-key.md)
+* "`T8aD +0x04` bit `0x02` selects premultiplied alpha" → **refuted.**
+  Premultiplied requires `RGB ≤ A` everywhere; over 170 decoded textures the
+  flagged group violates it on a median **52.5 %** of pixels against **30.2 %**
+  unflagged — both far from premultiplied, and the flagged group *further*.
+  [`ui-paint-order-key.md`](structures/ui-paint-order-key.md)