re(ui): scale 0 means collapsed, not "unset" -- stop drawing it full size

blit() and fill_quad() both opened with

    let sx_pct = if kf.scale_x == 0 { 100 } else { kf.scale_x };

so an element whose pose is collapsed to nothing rendered at FULL SIZE.
I first described this as "a 1-pixel sliver", reading the .max(1) in the
size arithmetic two lines below -- the guard above it meant .max(1) never
saw a zero. Read the whole function, not the lines you went looking for.

Control run before the change: if 0 meant "unset", something on the disc
would use it throughout. Nothing does.

  elements with a keyframe group                    15 493
  at least one keyframe at scale 0                   2 166
  EVERY keyframe at scale 0                              0
  zero is a transient the element grows out of       1 762

ptlogo_eff3.t32 runs 0% -> 200%. An "unset" marker no element ever uses
throughout is not a marker.

Both functions now return without drawing when either scale is 0.

Reach: 24 of 24 renders byte-identical across GP_TITLE (all 16 builds),
GP_PAUSE_MENU and GP_OPTIONS -- additive on every screen the port needs.
195 elements have a guessed rest pose at scale 0 and 126 of those have a
non-zero alpha, so the old code painted them; all 126 are in
GP_READY_ROOM.pak, which S1 already declared a no-go. They are
tactical-map sprites at sx=0/sy=100, mid-horizontal-wipe, that the
coercion drew full-width.

So: a real correctness fix with no visible effect on the menu port.
Suite green, 122 passed / 0 failed across 3 suites at commit time.

docs/re/METHOD.md

@@ -408,3 +408,14 @@ agent's loop prompt, i.e. nowhere durable. See [`README.md`](README.md) for the
   a heredoc two lines later wrote nowhere — while the `echo` after it still
   printed "written". A success message after a failed redirect is not a success.
   Use `(setsid nohup … &)` and keep the file-writing in its own call.
+* **Read the whole function before describing what it does.** I wrote "a
+  zero-scale pose paints a 1-pixel sliver" into a tool's docstring, from the
+  `.max(1)` in the size arithmetic. Two lines above it, a guard coerced
+  `scale == 0` to **100 %**, so the `.max(1)` never saw a zero and the element
+  rendered at *full size* — a much larger bug than the one I described. I had
+  read the lines I went looking for and stopped.
+* **Never diff a file a background job might still be writing.** A comparison
+  loop over an output directory reported `GP_TITLE` build 6 as changed; the pixel
+  diff was then all zeros, because `cmp` had read a half-written PNG. Compare
+  only after the job signals done, or skip the newest file. A "difference" that
+  vanishes on re-examination is usually a race, not a flake.

docs/re/REFUTED.md

@@ -408,3 +408,8 @@ neighbourhood, not just the line.
   makes `palogo_anima_eff` alone invisible while `gamearts_eff` and `seta_eff`
   stay lit. Capture box-mean ratios (0.717 / 0.723 / **0.772**) go the other way
   too. [`structures/ui-resting-pose.md`](structures/ui-resting-pose.md)
+* "a keyframe `scale` of 0 means *unset*, so render at 100 %" → **refuted by a
+  disc-wide control.** 2 166 elements have a zero-scale keyframe and **not one is
+  zero on every keyframe**, while 1 762 grow back out of zero (`ptlogo_eff3.t32`
+  runs 0 % → 200 %). Zero means collapsed; the renderer now draws nothing.
+  [`structures/ui-rat-layout.md`](structures/ui-rat-layout.md)

docs/re/data/zero-scale-census.txt

@@ -0,0 +1,24 @@
+# tools/re-capture/zero_scale_census.py -- 2026-08-28
+
+elements with a keyframe group:                 15493
+  at least one keyframe at scale 0:             2166
+  EVERY keyframe at scale 0 (would vanish):     0
+  zero is a transient the element grows out of: 1762
+rest poses with scale 0%%: 195
+  ... of which alpha > 0, so blit paints a 1-pixel sliver: 126
+  ... alpha == 0, harmless:                                69
+
+by archive:
+   126  GP_READY_ROOM.pak
+
+examples (pak, entry, element idx, name, sx, sy, alpha):
+   ('GP_READY_ROOM.pak', 0, 2, 'pbacropolis.t32', 0, 100, 255)
+   ('GP_READY_ROOM.pak', 0, 6, 'pbFriendly.t32', 0, 100, 255)
+   ('GP_READY_ROOM.pak', 0, 10, 'pbFriendly.t32', 0, 100, 255)
+   ('GP_READY_ROOM.pak', 0, 14, 'pbFriendly.t32', 0, 100, 255)
+   ('GP_READY_ROOM.pak', 7, 2, 'pbacropolis.t32', 0, 100, 255)
+   ('GP_READY_ROOM.pak', 7, 6, 'pbFriendly.t32', 0, 100, 255)
+   ('GP_READY_ROOM.pak', 7, 10, 'pbFriendly.t32', 0, 100, 255)
+   ('GP_READY_ROOM.pak', 7, 14, 'pbFriendly.t32', 0, 100, 255)
+   ('GP_READY_ROOM.pak', 74, 2, 'pbr_enemy.t32', 0, 100, 255)
+   ('GP_READY_ROOM.pak', 76, 2, 'pbr_enemy.t32', 0, 100, 255)

docs/re/structures/ui-rat-layout.md

@@ -711,3 +711,48 @@ full cycle length has to come from measurement.
 
 ❔ The word at `+0x004` is **not** the keyframe count: `0x003c0000` (60) with 8
 keyframes here, `0x001e0000` (30) with 3 in `ptloop01.rat`/`ptloop02.rat`.
+
+## ✅ `scale = 0` means COLLAPSED, not "unset" (2026-08-28)
+
+`ui_layout::blit` and `fill_quad` both used to open with
+
+```rust
+let sx_pct = if kf.scale_x == 0 { 100 } else { kf.scale_x };
+```
+
+so an element whose pose is *collapsed to nothing* was drawn at **full size**.
+(The `.max(1)` two lines below never saw a zero — see [METHOD.md](../METHOD.md).)
+
+### The control, run before the change
+
+If `0` meant "unset", some element would use it throughout. Over the disc
+([`data/zero-scale-census.txt`](../data/zero-scale-census.txt)):
+
+| | |
+|---|---|
+| elements with a keyframe group | 15 493 |
+| at least one keyframe at scale 0 | 2 166 |
+| **every** keyframe at scale 0 | **0** |
+| zero is a transient the element grows back out of | **1 762** |
+
+Not one element on the disc is zero-scale throughout, and 1 762 grow out of zero
+— `ptlogo_eff3.t32` runs `0 % → 200 %`. **An "unset" marker that nothing ever
+uses throughout is not a marker.** Zero means zero.
+
+### The fix, and its reach
+
+Both functions now return without drawing when either scale is 0.
+
+* **Renders: 24 of 24 byte-identical** across `GP_TITLE` (all 16 builds),
+  `GP_PAUSE_MENU` and `GP_OPTIONS`. The change is additive
+  on every screen the port needs.
+* 195 elements have a guessed rest pose at scale 0; **126 of those had a non-zero
+  alpha**, so the old code painted them at full size.
+* ⚠️ **All 126 are in `GP_READY_ROOM.pak`** — the archive
+  [S1](../ready-room-probe.md) already declared a no-go. They are tactical-map
+  sprites (`pbacropolis`, `pbFriendly`, `pbr_enemy`) at `sx=0, sy=100`, i.e.
+  mid-horizontal-wipe, which the old coercion drew full-width.
+
+🟡 So this is a **real correctness fix with no visible effect on the menu port**.
+It matters for anything that later reads `GP_READY_ROOM`, and it removes a case
+where the renderer contradicted the format.