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/structures/ui-paint-order-key.md

@@ -582,3 +582,45 @@ authoring intent rather than by name, which is a description, not a decode.
 `T8aD`, validated 17/18 on build 4 against the child order. The one mismatch is
 the known `pteff04.t32` → registered as `8AX` case, so this is the **element**
 (`opt `) name, not the sprite's registered name.
+
+### 🔴 Premultiplied alpha — a fourth candidate, refuted
+
+A per-sprite flag controlling **premultiplied vs straight alpha** would matter
+enormously to a port (it changes the blend equation) and has a sharp static
+signature: a premultiplied texture has `RGB ≤ A` everywhere.
+[`tools/re-capture/eff_bit_alpha_test.py`](../../../tools/re-capture/eff_bit_alpha_test.py),
+over the 170 decoded `GP_TITLE` textures that pair to a flag word
+([data](../data/eff-bit-alpha-test.txt)):
+
+| group | n | mean %(RGB>A) | median |
+|---|---|---|---|
+| bit SET | 61 | **55.5** | 52.5 |
+| bit clear | 109 | **33.7** | 30.2 |
+
+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 is left is a weak association: flagged sprites carry more bright-RGB /
+low-alpha pixels, which is what glow art looks like. But the distributions
+overlap badly — the best single threshold classifies **76.5 %** against a
+**64.1 %** base rate, a 12-point lift. That is a tendency, not a rule, and
+nothing to build on.
+
+### ⚠️ One re-test worth remembering
+
+"`0x02` selects an additive blend" was refuted by *"blending those sprites
+additively worsens every measure against the capture"* — but that ran against a
+title render which has since been fixed twice (the `rest_plateau` bug, and the
+`8AX` background our composer drops). The refutation may still stand; it was
+simply measured through a renderer with known other errors. If `ui_layout::blit`
+ever gains additive blending, it is worth one re-run.
+
+### 🛑 Parking this field
+
+Four candidate meanings are now dead — additive blend, `eff` name (both
+directions), transient element, premultiplied alpha — and none of the four
+searches produced a positive account. The bit blocks nothing: the port's screens
+composite at 0.947 correlation against a capture without it. **Parked**, with the
+search space narrowed and the attribution method (child order, not size) left
+sound for whoever picks it up.