test(ui-theme): resolve elevated surfaces from the palette, not from rebinds

The check added last commit derived its elevated set from the sheets that
already rebind, which cannot catch the omission it targets: such a set only
confirms what someone already remembered, and a surface nobody has rebound
yet defines itself as unelevated. Review found the case that proves it —
TodoPanel scrolls in .list on a --dsw-specific-tip card, the same dark rung
as the menu surface, unrebound and with the derived check green.

Resolves the set from the palette's own dark elevation ladder instead: the
surface tokens whose dark value lands on bg-layer-2 or bg-layer-3, which is
the step the l1/l2 split encodes. A new palette token on an elevated rung is
in scope the moment it is defined.

Scope is by token family rather than geometry: only --dsw-alias-bg-* and
--dsw-specific-* name a surface. The button, interactive, and markdown
families reach the same rungs while naming a control or an inline span that
no scroll container renders a bar against, and shape cannot separate them
since a floating button carries a radius, a shadow, and a fixed size —
ChatView's .toBottom pill was the false positive that showed this.

Adds the missing TodoPanel rebind. Mutation-checked all four rebinds in
turn: each is named with its surface. The palette anchoring has its own
control — narrowing the family pattern turns it red on --dsw-specific-menu.

packages/client/ui-conversation/src/client/skeleton/TodoPanel.module.css

@@ -11,6 +11,13 @@
   border: 1px solid var(--dsw-alias-border-l1);
   border-radius: 14px;
   background: var(--dsw-specific-tip);
+  /* Elevated surface: `--dsw-specific-tip` is the same dark rung as the menu
+     surface, and `.list` scrolls inside this card, so the thumb takes the l2
+     elevation tokens. Declared here because the elevation belongs to the
+     surface, and the custom properties inherit down to `.list` (see ui-theme
+     styles/scrollbar.css for the rebinding contract). */
+  --dsh-scrollbar-thumb: var(--dsw-alias-scrollbar-bg-l2);
+  --dsh-scrollbar-thumb-hover: var(--dsw-alias-scrollbar-hover-l2);
 }
 
 .body {