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/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.