re(ui): a RATC child's name is stated, not inferred -- and it was hiding every menu background

`ratc::parse` named each child by scanning backwards for the last printable run
of bytes before its magic. The format states the name explicitly instead, in an
`opt ` block: `"opt " | BE32 len | name | NUL | 3 bytes | magic` -- the same
block `ui_layout::opt_link` already read for a button's focus link.

The scan agrees with it 17 918 times out of 17 942 and is wrong 24 times, every
one the same failure: the 3 trailing payload bytes are themselves printable and
beat the real name. For `pteff05.t32` those bytes are `38 41 58` = `8AX`, so the
full-resolution background of all five menu screens registered under a name no
element declares, resolved to no sprite, and `compose` dropped it through an
early `continue` that -- unlike the two arms above it -- records nothing. The
screen lost its background and `screen render` still reported "all resolved".

`8AX` was never a name. Docs that treated it as one are corrected here.

Disc-wide, and the control is the 17 918 the scan already got right: the `opt `
reading reproduces every one of them. Effect on the five screens is the
signature of the same art at twice the resolution -- mean brightness unmoved,
high-frequency detail x1.15..x1.30 -- which is what the separately-measured
`ui-8ax-fullres-background` result said the game draws.

Also closes a long-standing dangling reference: `pmbase.t32`, recorded as "on
the disc nowhere", is the `GP_STAGE_CLEAR` child the scan called `8AX`. RATC
sibling references now resolve 10 148 of 10 148.

Verified: 114/114 sylpheed-formats unit tests (including two new ones pinning
the `8AX` case byte for byte and the no-block fallback), and every disc-gated
integration suite in sylpheed-formats/sylpheed-cli.

docs/port/HANDOFF.md

@@ -261,7 +261,26 @@ authored version can be deleted.
   setting rather than bake in.
   [`structures/ui-render-tone-curve.md`](../re/structures/ui-render-tone-curve.md)
 
-* 🟡 **`screen render` silently drops one full-screen element per screen — and
+* ✅ **FIXED 2026-08-29 — the dropped background was a name-decoding defect, and
+  `screen render` now draws it.** The reference composites for all five screens
+  changed; regenerate anything you diffed against before that date. Cause: a
+  RATC child's name is stated by an **`opt ` block** immediately before it
+  (`"opt " | BE32 len | name | NUL | 3 bytes | magic`), and our parser instead
+  guessed it from the last printable run of bytes. For this one child the 3
+  trailing bytes are `38 41 58` = `"8AX"`, which beat the real name
+  `pteff05.t32`. **`8AX` was never a name** — earlier text on this page treating
+  it as one was wrong. Disc-wide: 18 002 children, 17 918 already agreed with the
+  `opt ` reading and **24 did not**, every one the same 3-byte-tail failure.
+  Effect on your five screens: mean brightness unmoved, high-frequency detail
+  **×1.15…×1.30** — the same art at twice the resolution, which is exactly what
+  [8AX](../re/structures/ui-8ax-fullres-background.md) said the game draws.
+  ⚠️ Both backgrounds are now drawn (`ptbase` upscaled, then the full-res one over
+  it): correct, but wasted fill. Draw only the full-res one, taking its *timing*
+  from `ptbase`'s element, which carries the keyframes.
+  [`structures/ratc-child-names.md`](../re/structures/ratc-child-names.md)
+  <details><summary>the original entry, kept because its reasoning still stands</summary>
+
+  🟡 **`screen render` silently drops one full-screen element per screen — and
   you must NOT simply draw it.** Auditing what the composer omits on your five
   screens: everything is accounted for (`kind & 0x4` ghost instances, `.prm`
   primitives, `loop*` animations) except **`pteff04.t32`** on the title and
@@ -287,9 +306,11 @@ authored version can be deleted.
   full-screen layer over an identical one costs fill and hides later changes; and
   note `ptbase`'s element is the one carrying the keyframes, so you need its
   timing with `8AX`'s pixels.
-  ⚠️ It does not show whether `ptbase` is *also* drawn underneath — `8AX` is
+  ⚠️ It does not show whether `ptbase` is *also* drawn underneath — the full-res
+  background is
   ~86 % opaque and would hide it either way.
   [`structures/ui-8ax-fullres-background.md`](../re/structures/ui-8ax-fullres-background.md)
+  </details>
 
 * ✅ **Paint order: your exposure is two element pairs, on one screen.** We use
   an order *measured from the running game* where one exists and a derived order