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/port/HANDOFF.md

@@ -120,7 +120,7 @@ authored version can be deleted.
   ⚠️ **Correlation only — untested.** Nothing yet shows it *means* additive; the
   test is to blend bit-`0x02` sprites additively and re-correlate the title
   against the capture. Note `ptlogo_back2eff` is `0x8830` despite its name, so
-  this is a field and not a naming pattern. ❔ Not diagnosed — but two candidate meanings are now dead: **not** "the name contains `eff`" — and not even the one-way "bit ⇒ `eff` name", which held 10/10 on the title but fails on **2 657 of 4 995** bit-set sprites disc-wide (`P(eff|set) = 0.468`). What survives is a 3.3× association, and the counterexamples are rings, glows and lights — effect-like art without the naming convention and **not** "the element is transient" (`pteff03`/`pteff03a` carry the bit and persist to `t=250`). ⚠️ Practical note for an asset pipeline: `T8aD` headers appear in **RATC child order** (18/18 verified on build 4 against decoded dimensions), which is the only sound way to attribute a header to a name when two sprites share a size — and two here do.
+  this is a field and not a naming pattern. ❔ Not diagnosed — but two candidate meanings are now dead: **not** "the name contains `eff`" — and not even the one-way "bit ⇒ `eff` name", which held 10/10 on the title but fails on **2 657 of 4 995** bit-set sprites disc-wide (`P(eff|set) = 0.468`). What survives is a 3.3× association, and the counterexamples are rings, glows and lights — effect-like art without the naming convention and **not** "the element is transient" (`pteff03`/`pteff03a` carry the bit and persist to `t=250`). 🛑 **Parked** — four candidate meanings are now dead (additive blend; `eff` name, both directions; transient element; **premultiplied alpha**, refuted because flagged sprites violate `RGB ≤ A` *more* than unflagged, 55.5 % vs 33.7 %) and none produced a positive account. It blocks nothing — your screens composite at 0.947 correlation against a capture without it. ⚠️ Practical note for an asset pipeline: `T8aD` headers appear in **RATC child order** (18/18 verified on build 4 against decoded dimensions), which is the only sound way to attribute a header to a name when two sprites share a size — and two here do.
   ⚠️ **The rotated draw is NOT the swoosh.** It was identified as the swoosh by
   elimination; that is **refuted** — its quads span y −209…925 in screen space,
   the swoosh is a band at y 126…360. ✅ **It is the two `ptloop` sweeps**

docs/re/METHOD.md

@@ -700,3 +700,15 @@ agent's loop prompt, i.e. nowhere durable. See [`README.md`](README.md) for the
   `px_bunk_line` — rings, glows, lights, thin lines. That the exceptions are all
   effect-like artwork *without* the naming convention says more about the field
   than the rule it broke did.
+* **Report a classifier's lift over its base rate, not its accuracy.** The
+  `0x02` bit's association with bright-RGB/low-alpha art gives a best single
+  threshold of 76.5 % accuracy — which sounds like a finding until you notice
+  that always guessing the majority class scores 64.1 %. The lift is 12 points,
+  the distributions overlap, and it is a tendency rather than a rule. An accuracy
+  quoted without its base rate is not interpretable.
+* **Park a field after N failed hypotheses, and say what was eliminated.** Four
+  candidate meanings for one flag bit were each refuted at a cost of roughly an
+  iteration apiece, and the bit blocks nothing measurable. Stopping is the right
+  call, but only if the negative space is written down — additive blend, name,
+  lifetime, premultiplied alpha — so the next attempt starts where this one ended
+  rather than at the beginning.

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)

docs/re/data/eff-bit-alpha-test.txt

@@ -0,0 +1,7 @@
+matched 170 decoded textures to a T8aD 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 would require ~0% for the flagged group -> REFUTED
+  best single-threshold accuracy: 76.5% at %(RGB>A) > 83.5
+  (base rate, always-guess-majority: 64.1%)

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.